Advanced nitrox divers graduate with bull sharks in Thailand

Koh Tao, Thailand - Big Blue Tech celebrates the graduation of Yvonne Fries, Helen Artal, Thomas Hallstrom and Duncan Tyler from a TDI Advanced Nitrox course conducted by TDI Instructor James Thornton-Allan and assisted by Andrew Cavell and Ash Dunn over various dive sites on Koh Tao Island in Thailand.

The TDI Advanced Nitrox course is designed to orientate the student about rich or high mixes of oxygen and their advantages while wearing technical diving gear. The use of low mixes to advance deep diving and the use of high mixes to add extra conservatism to optional stops during the dive.

The students learned about carrying a decompression cylinder, oxygen handling and analysis and vairous other skills. The final dives were conducted using nitrox to allow the diver to flow through a no-decompression schedule switching to different mixes of nitrox the shallower the dive went. This course certifies the diver to delve to 40m using up to 100% oxygen depending on the depth and if the situation is warranted.

The final dive was also held at Chmphon Pinnacle where we were met by Bull Sharks that live at that dive site creating an exciting atmosphere for learning and diving.

The students were issued certification after and exam and progress on to their TDI Decompression Procedures course tomorrow with a return to Chumphon Pinnacle for some more dive time with the sharks. You can read more about the Advanced Nitrox course here: TDI Advanced Nitrox Diver Course

During diver training, dive students are normally drilled to avoid diving beyond 130 feet / 39 meters. However this depth limit recommended by most of the training agencies is not forged in stone. Historically, it appears that it probably emerged from the U.S. Navy, possibly as a result of equipment limitations at that time, and the work restrictions imposed by the relatively short no-stop times available at greater depths.

An increasing number of divers dive beyond the 130-foot limit, some routinely and others occasionally. The advent of dive computers has negated much of the decompression penalty and dive restrictions previously associated with deep diving, and has no doubt encouraged the current trend. In addition, the increased availability of a variety of gas mixtures has enabled more divers to venture deeper and deeper.

Deep diving demands vast amounts of knowledge, experience and discipline, as well as appropriate preparation and equipment, since deep diving is fraught with potential hazards.

An old friend of mine used to teach diving at a tropical resort. The instructors routinely dived on air to depths approaching 300 ft (90m) and beyond on their days off. During such a dive, one instructor became unconscious at about 200 ft (60m) without obvious warning. He fell away and out of reach of the others before anyone could get it together to do anything. His body was never recovered.

Elsewhere, another diver diving at just over 165 ft (50m) on air on a wreck was seen to become unconscious and to convulse. Luckily his buddies managed to rescue and resuscitate him.

These are not isolated stories, and there are many similar reports involving deep air dives and mixed gas dives.

Unconsciousness underwater is often associated with deep diving accident reports. It usually results in drowning. A number of conditions can cause a diver to lose consciousness underwater.

All of which are exacerbated by depth. Blackout underwater may not be due to a single cause, but may result from a combination of physiological or physical factors.

Nitrogen narcosis can become a very serious adversary on deep air dives. Although we can acclimatize ourselves to the affects of narcosis to some extent by regular exposure to depth, it can still sneak up and very quickly overcome us when we don’t expect it. Although conventional wisdom states that the narcosis appears on arrival at a particular depth and usually does not worsen with continued exposure at that particular depth, many divers are aware that it can quickly be precipitated by exertion or stress at depth, without further descent.

Divers who have had to quickly deal with a problem at 200 ft (60m) on air realize the extreme difficulty of reacting rapidly and appropriately. Sometimes the mind-numbing effects of narcosis can strike suddenly and make appropriate reactions almost impossible. Extremely high levels of stress can be precipitated instantaneously and, unless controlled, panic and injury are likely results. Narcosis may be the direct cause of unconsciousness in a diver at depths somewhere in excess of 200 ft. Narcosis can be reduced by using certain gas mixtures. However, this involves the appropriate equipment, preparation, training and care since new potential hazards are introduced.

Carbon dioxide acts as a respiratory stimulant and can cause depression of the central nervous system (CNS). The effect depends on the level of carbon dioxide in the blood.

Hypercapnia increases narcosis and the likelihood of CNS oxygen toxicity. In addition, it may increase heat loss, alter heart rhythm and predispose to decompression illness. If the carbon dioxide level gets too high, and it can on deep scuba dives — especially if a diver is very anxious and / or exerting him/herself — the diver may go unconscious without warning. Certain divers are more susceptible to severe hypercapnia for a variety of reasons and are therefore more at risk.

When divers breathe oxygen at partial pressures greater than about 1.5 atmospheres (ata), it may rapidly exert a toxic effect on the brain. A diver breathing air at a depth of around 200 ft is exposed to an oxygen partial pressure of 1.5 ata. CNS toxicity is a limiting factor and a very real danger in deep diving since it can cause a diver to convulse and/or become unconscious with little or no warning. The likelihood of CNS oxygen toxicity increases with exposure time, cold, exertion and hypercapnia, and the depth and time of onset can vary greatly between individuals and from dive to dive.

The high nitrogen load accumulated by the “fast” and “medium” body tissues during a deep air dive can cause substantial bubble formation during or after ascent unless the decompression is properly controlled and conducted. Some of these bubbles may form in or enter the arterial circulation and cause neurological problems. This mechanism may be responsible for some underwater blackouts during ascents from deep dives.

Various data indicate that deeper diving is associated with a substantially increased risk of decompression illness. This risk appears to increase at depths beyond about 80 ft (24m). In addition, using a dive computer to guide decompression from deep air dives appears to increase the risk further due to the greater dive times allowed and the increased unreliability of the algorithms at depth. More and more divers have adopted the use of various gas mixtures in the belief that it will reduce the risk of decompression illness. However, recompression centers still treat a significant number of these divers.

Certain studies suggest that microbubbles are often present after dives, particularly deep dives, especially if ascent has not been appropriately executed but even after what is generally considered to be a safe ascent. Some hyperbaric specialists fear that microbubbles, although asymptomatic, may cause cumulative neurological damage in divers. However, to date, the evidence does not appear to be consistent.

Unless adequately prepared for, deep diving carries a higher likelihood of an air supply emergency. Increased ambient pressure means increased air consumption. In addition, narcosis may hinder a diver’s ability to properly monitor and manage the air supply. Despite the improvements and superior performance of much of the modern diving equipment, malfunctions do occur. The deep divers who value their hides ensure that they have adequate backups of various essential pieces of equipment, including an independent and adequate air supply.

Buoyancy compensation can sometimes become a critical factor on deep dives, especially in cold water where greater insulation is required. Unless compression of the exposure suit is adequately compensated for by BC or dry suit inflation, a diver may become very negatively buoyant at depth.

Wreck divers may sometimes prefer to be negatively buoyant, but problems can develop if the air supply is low and the diver needs to ascend fairly quickly.

Various experiments have demonstrated that, at low cylinder pressures, it is sometimes impossible to inflate a BC (or dry suit) at depths approaching 130 ft, especially while breathing simultaneously from the regulator. This problem would be magnified at greater depths. At times, a negatively buoyant diver who is low on air may find it difficult, or even impossible, to ascend without ditching their weight belt. If the weight belt is ditched, it is unlikely the diver will make it to the decompression line to get some extra air and perform any necessary stops.

Some divers routinely dive to depths in excess of 165 feet/50 meters on air, although over recent years gas mixtures such as heliox and trimix have become far more commonly used for very deep diving as they are less narcotic. These divers are often, but not always, conversant with the substantial risks and demands of these dives and choose to push the limits for their own reasons. Such divers are usually well equipped and well prepared for the dives. Most usually manage to get away with diving to these depths with no apparent problems, others do not. Some of the unfortunate ones are left with permanent disability; others die.

On the other hand, there is the “occasional” deep diver. These divers are generally less experienced than regular deep divers, are on a dive trip with a group, and are drawn into diving deeper than they normally do because of the more relaxed holiday atmosphere and because “everyone’s doing it.” Such divers are often not sufficiently trained, mentally prepared and appropriately equipped to deal with a problem should it occur on a deep dive.

It becomes obvious that there is no safe depth limit that applies to all divers all of the time. A diver’s ability to cope with depth depends on a number of highly variable factors. The depth of the onset of the effects of the exotic cocktail of elevated pressures of nitrogen, carbon dioxide and oxygen, coupled with the sensory deprivation and stress associated with diving, are not always predictable. A dive to 80 feet in cold, dirty water can be far more hazardous than a dive to twice the depth in warm, clear waters. Factors such as visibility, water temperature and diver experience and preparedness greatly affect a diver’s comfort and safety, rather than depth alone.

Divers in remote locations must also be aware of the complications associated with medical evacuation. These can include significant delays in retrieval due to lack of current availability of an aircraft and and/or medical team, the distances involved, as well as the accessibility of some airstrips in darkness or adverse weather conditions. Such delays can impact the amount and the effectiveness of the subsequent recompression treatment, and the likelihood of residual injury.

In addition, once a diver has been evacuated and/or treated for DCI, they will be advised to avoid air travel or driving to altitude for between three days and six weeks post treatment to avoid recurrence of symptoms. This can certainly impinge upon the diver’s travel and work commitments.

As with many things in life, one must balance the risks against the benefits and make a decision. However, it is essential to have a real understanding and appreciation of the risks.

Like he did almost every week, Correy Fedor went scuba diving with his pals from Any Water Sports in San Jose.

But something horrible happened to the experienced diver Thursday morning. The 22-year-old Fedor had some sort of trouble, and he became separated from his two friends. They surfaced. He didn’t. And when he finally was found, he was dead.

Today, his friends and family gathered in San Jose to mourn “one of the good guys,” said Fedor’s aunt, Jill Perry. “He was one of those great kids. We may never know what happened to him.”

Fedor, along with Frank Barry who owns Any Water Sports, and another friend, Scott Chapman, were scuba diving off Monastery Beach in Monterey County on Thursday before work starting at 7:30 a.m., according to Barry’s wife, Ginny Barry.

State Park Sector Superintendent Dana Jones said Fedor was doing a “technical dive” down 250 feet — which is quite deep for amateur divers — but something Fedor had done before.

Fedor’s family said his equipment has been checked out, and nothing was malfunctioning. He had no health problems that his family knew about. According to the nonprofit group Divers Alert Network, deep diving can be fraught with potential hazards including something called nitrogen narcosis,a condition that can sneak up quickly and cause the diver to not think clearly. The Monterey County coroner’s office, however, has not yet released the cause of death.

Recounting the story told to her by her husband, Ginny Barry said the trio were making their “scheduled stops” back up to surface — something that’s necessary because it’s impossible to shoot straight back up to the surface.”They meticulously planned this dive,” Barry said. “But it was dark and overcast. Frank checked his compass and tapped Correy on the arm and pointed in the direction they were traveling. He assumed Correy was behind him and making his safety stops.”

When Frank Barry checked again next to him, Fedor wasn’t there, but he thought his friend was still following the plan. Barry and Chapman reached the surface.

When they couldn’t find Fedor — a University of California-Santa Cruz student studying anthropology — Chapman called 911 and Barry scoured the water. Fedor worked in Barry’s shop as a diving tech, and also on Barry’s charter boat in Monterey as a deckhand and dive master, Ginny Barry said.

Finally, her husband found Fedor facedown in the water and tried to revive him, rangers said. Emergency crews rushed on scene and performed CPR until 10:30 a.m. to no avail.

Fedor’s aunt said her nephew, a 2005 Santa Teresa High School graduate, was beloved in the scuba community. Fedor’s parents, Vanessa and Joe, along with brothers Jarred, 26, and Preston, 29, have been reading scuba blogs, learning how much divers connected to the happy-go-lucky Fedor, who also loved hiking, rock climbing and fishing.

“He and his father were planning one last fishing trip before school started,” Perry said. “They were supposed to put it on the calendar last night.”

Since 2004, 15 divers have died in Monterey County waters at spots including Point Lobos, Lovers Point in Pacific Grove and San Carlos Beach near Cannery Row, according to the Divers Alert Network medical research department. Two deaths reportedly occurred at Monastery Beach in that time period.

This is the second time this year a San Jose man has died after scuba diving in the Monterey area, although the other death was a different beach. In early August, Alec Piplani, 49, had trouble breathing after entering the water off McAbee Beach on Cannery, and was later pronounced dead at the hospital.

“I’m a diver. I do underwater construction.”

As simple as it sounds to say, it can be one of the most amazing, scary and thrilling occupations all rolled into one. For one Stuttgart resident it’s what has fulfilled his expectations in a job.

Brent Saranie, who grew up in Stuttgart, has now returned to start a family, but what he does when he is away is what makes one part of his life so interesting — what he does for a living.
He has worked on a multitude of things — all underwater — bridges, pipelines, nuclear plants, dams, drinking water intakes, boats, ships, salvage, steel and paper mills.

Saranie started his underwater career eight years ago when he decided to go to school for the occupation.

“I used to mortgage houses — went to college — mortgaged houses,” he explained. “I wanted to go find a way to make a living and see a little bit of the planet — see different things. And I wanted to come back one day and be able to tell my kids about it and go ‘that’s what I did.’ I scuba dove. I was spear fishing up in Hot Springs all the time and just picked up the phone book one day and asked how you get hired. They told me I had to go to school.

So Saranie did just that. He went to Houston and started at Ocean Corporation. It took 900 hours. They trained four days a week 8 a.m. to 5 p.m. for seven months.

You can see the excitement in his eyes, the facial expressions and the ever moving hands as he tells about the things he gets to see — things most never see in a lifetime.

“My favorite part is seeing the things that you are going to work on — it’s cool,” he says. “You get to see cool stuff. You see robots that go down to 3,000 feet deep and turn bolts and video of the creatures that are down there. Just being around things that you don’t normally see. Another thing I like is working with people from around the world. You work with people from all over and dealing with the people every day — it is interesting, you learn a lot. It is fun, but at the same time, it is nerve racking.”

Another part of the job he enjoys is getting numerous days off, which everyone can relate to as being a good aspect of any job.

The only part of the job he said he could do without is being away from his family. He is recently married and also became a father, so he misses his family quite a bit when he is out on a job.

“Being away from family and that is about it,” he says. “Because it’s fun when she [his wife Melissa] goes with me — you know when we go inland — we have a blast.”

Saranie works for Veolia Special Services. The farthest place he has been for a job is Zhanjiang, China.
Surface diving
Like most occupations, there are different facets to the job. In Saranie’s case, there is different diving. The first is surface diving where you are breathing air.
“You can go down about 180 feet on that. At that depth, you have a 35-minute bottom time — on air. Then you have five hours of decompression. I wear my diving hat and a hot water suit,” he explains.

The hot water suit takes hot water from a heater above, which pumps down and disperses water around his body. It is not an enclosed suit, which means it flushes water out also.
Diving in nuclear plants

In a nuclear plant the diver wears a chiller suit and it’s a closed circuit where everything is being pumped back to the surface.

“That is a whole different ballgame talking about it and to see it,” he says. “The water acts as an insulator to protect your nuclear rods and to keep your radiation away from everything. Some of the tanks are 40 feet deep and they can’t drain them so they will send us in.”
The water is hot — 104 degrees — hot.

“So what your do is you wear an encapsulated suit, everything is sealed and the hat seals to the suit,” Saranie explains.

Your two options are to wear the suit or have your body packed in ice, although the ice doesn’t last that long.
Gas diving
After the initial 180 feet, helium is mixed with oxygen.

“Because we breathe 21 percent air here — air is toxic when your parcel pressure goes up,” Saranie says. “When the pressure on your body is forcing the oxygen into your muscles it becomes toxic and you can have seizures and stuff.”

The only downfall of mixing is the helium robs the body of heat in the body’s core temperature.
“So when you exhale it exhales all your heat. So they always make you wear hot water, you get chilled really bad. The deepest I have been on gas, jumping off the side of a boat, is 242 feet and I had a five and half hour decompression. I had so many hours in the water decompressing then I went into a decompression chamber.”
Saturation diving and the chamber

In saturation diving, which is what Saranie is doing now, he lives in a decompression chamber.
The chamber — the one he currently stays in —is seven and half feet in diameter and 20 feet long. There are six guys living there. The bathroom is bolted on and it is eight feet long and seven feet wide.

“Then we have what they call a bell — the bell is how we travel back and forth to work. The saturation system is bolted to the top of the ship so it is in the dry — everything is in the dry,” he says. “They press you down to the depth pressure and hold you there. When they want you to work, you crawl into the bell, it seals up with airtight doors and they launch it over the side or through the center of the ship.”

When the desired depth is hit, water will start to flood the bell and divers let the operators know that they need to stop to let the bell partners get ready.

“You tell them to stop and you lift the door up and tie it off,” he says.

There are two divers in the bell and they help one another get dressed to begin the work.
Bell partner
“He is your person,” Saranie explains about his bell partners. “Like, if you get an infection, or if you get burned or cut or if he has ear infection — you are putting the drops in his ears and he is helping you. I got burned the last time and he was putting the cream on me every night. Ya’ll are friends — that is your person. If you have to talk to somebody, if you have some issues that you need to go over, you have to discuss it with your bell partner — you have a very close relationship.”

The relationship has to be strong because they depend on one another.

“You put up with him and he puts up with you. We stay in there for 28 days and you work for 21 and then you decompress for seven. So it is seven days of sitting around,” he says.
The waiting time

Saranie says he can read a 400-page novel in one day. His last stretch he read a total of 12 books and finished a crossword and Sudoku book

“And we take movies down there and your PSP [PlayStation Portable] works down there and my phone works — I can text Melissa,” he said.

His food is brought to him, the toilets are flushed for him and the water is turned on for him.
“Everything is handled from the outside for you in there,” he says. “All your living is monitored.”
Do you get scared?

“Yeah, you’re not human not to get scared,” he said. “When you get below 500 feet, life changes.”
Practicing in school will teach you, he explains, but not like on-the-job training.

“You practice in school. Schools have different things, some dive in tanks because they can control everything, and some take you out to a bay. It is a good stepping stone but you don’t really get into it — most of it is on the job,” Saranie says.

When the diver gets below 500 feet, half his body will be cold and half will be hot.

“Below 500 feet is miserable,” he says.

At 650, he explains, it is like walking around breathing through a straw all day.

“What you do is open your suit up and break all this out [chest area] and let the cold water hit my chest and the cold water slows your heart down,” he says.

He is being monitored at all times and operators can talk with him. He has a beacon strapped to his back and GPS monitors his every move.

“They can see where you are walking, they know how deep you are, they know where the crane is, where the boat is, where the bell is and, I mean, they know where you are at and they can tell you everything you need to know,” he said. “You got to get in your mental mind like George Burke said, that depth is tough.”

During penetration diving, Saranie has found himself scared. Penetration diving has only one way out — the diver is inside a pipe. So no matter what, he has to get himself out of the pipe before he can be brought back up. Saranie’s longest penetration was 2,800 feet.

Where you have seen my work
Saranie had worked on Nuclear One in Russellville. He was also a part of constructing a bridge going across Tampa Bay. He also has worked on the Las Vegas water system. Currently he is working in China and will continue to work there for six, possibly seven, more years. Four oil platforms have been built, but the company wants 10 more.

When asked if he sees himself doing anything else in life, with a hearty laugh he replies, “maybe on my days off.”

Saranie lives with his wife Melissa and daughter Nadia in Stuttgart.

Take a deep breath. You just filled your lungs with air, the common name for a gas mix of approximately 21 percent oxygen and 79 percent nitrogen (give or take less than 1 percent for trace gases like argon and helium). Without it, you’d keel over and die. But when it comes to diving, “Nobody ever said air was the best gas to breathe,” says technical diving pioneer Dick Rutkowski.

Any diver worthy of his C-card understands why–all that inert nitrogen does funny things in your body under pressure. Much of what you learned in open-water training is designed to mitigate the accumulation of nitrogen in your tissues to prevent decompression illness (DCI). And on dives below 100 feet, nitrogen starts to produce a narcotic haze that can become quite debilitating as you go deeper. Somewhere between 150 and 180 feet, most divers will be so narced that they are incapacitated.

Go beyond 180, and oxygen starts to be a problem. Somewhere between 190 and 220 feet, oxygen becomes toxic, resulting in sensory distortions and seizures that can be fatal under water.

Clearly, air has its limitations as a diving gas, particularly for divers who want to stay longer or go deeper than the traditional recreational diving limits. Which is why tech divers have long been experimenting with alternative blends in the search for a better diving gas.

Nitrox

The logic behind nitrox is simple: Replace some of the nitrogen with more oxygen. Less nitrogen in the tank means less nitrogen in the diver and fewer problems with DCS and narcosis.

There’s just one catch: When diving with enriched air, you have to monitor your oxygen exposure to avoid toxicity. There are two crucial factors to consider. The first is the relative percentage of oxygen (or PPO2) in your tank. The second factor is the time of exposure. The combination of the two tells us the total oxygen dose. An oxygen dose of 1.6 PPO2 for 45 minutes is recognized as the maximum safe limit for divers who aren’t working strenuously.

Fortunately for recreational divers, this dose limitation gives us a wide latitude to dive in the 60- to 130-foot range using gas mixtures from 32 to 55 percent oxygen. With proper planning, you can significantly increase the duration of your dives without any increased risk of decompression illness or narcosis and with an extremely low risk of oxygen toxicity.

Trimix

To go beyond traditional recreational depths, technical divers employ trimix, the general term for gas blends that replace much of the nitrogen and some of the oxygen with more benign inert gases, such as argon and helium.

Based on current research and practical experience, helium is the inert gas of choice. Its narcotic properties are negligible in comparison to nitrogen and it’s a thinner, more compressible gas that helps regulators work more efficiently at extreme depths. Helium-based mixtures allow properly trained and equipped divers to routinely go to 400 feet (and beyond) with a remarkable safety record.

Trimix divers custom blend their breathing gas to suit each dive, allowing them to more precisely control oxygen limits and more dramatically reduce narcosis. For example, let’s look at a 240-foot dive using 17/50 trimix. That’s 17 percent oxygen, 50 percent helium and 33 percent nitrogen. (When expressing percentages in a trimix gas, the oxygen is always stated first, followed by the helium. The balance of the gas is assumed to be nitrogen unless otherwise stated.) This dive yields a conservative PPO2 of 1.4 with an equivalent narcosis depth of only 57 feet. In other words, the diver would experience the same level of narcosis on this trimix dive as he would on an air dive to 57 feet.

The Cost of Deeper Diving

Of course, these benefits do not come without cost. The deeper the dive, the more complicated the dive plan and the gear configuration become. The first thing most recreational divers notice is all that redundant gear, particularly the twin back-mounted cylinders coupled together with an isolation manifold. The manifold allows the gases from the two tanks to “communicate” so that the diver can use the regulator attached to either tank while consuming the gas supply equally from both tanks. On deeper trimix dives, the oxygen content in the main cylinders may be too low to safely breathe at shallow depths. In these cases, the diver must also carry separate gas mixtures in additional tanks–one cylinder of travel gas (typically a nitrox blend of 32 to 40 percent oxygen) to breathe while passing through traditional recreational diving depths and another cylinder of decompression gas (either 80 percent or 100 percent oxygen) for shallow stops.

Decompression and Helium

While helium is extremely useful in combating the ill effects of nitrogen and oxygen at extreme depths, it’s not without its problems. Because it is a lighter, faster gas, divers load and unload helium more quickly than nitrogen. For some profiles, this requires slower ascents or deeper decompression stops to keep the helium from off-gassing too rapidly and causing DCI.

Finally, the major drawback to helium-based gases is cost. Helium supplies are tightly regulated, making the gas very expensive. The cost of filling a set of doubles with a trimix will typically range anywhere from $30 to $120, depending on the amount of helium used.

As you might also expect, trimix training is quite involved and quite expensive. Trimix courses typically require certification as an advanced open-water diver, an advanced nitrox diver and a decompression procedures diver as prerequisites. The course itself will range from $900 to $1,200 in the United States, depending on geographic location, and the cost is exclusive of gas fills, charter fees, etc. When all the expenses are totaled, divers starting a trimix class should anticipate spending between $1,500 and $2,000, assuming they already have all their own gear.

Calculating a Narcosis Depth

One of the most important criteria for selecting the right trimix blend is calculating a narcosis depth air equivalency–a comparison of the expected narcotic effect of a tri-mix blend at a given depth to a more easily understood air depth.

To accomplish this, we must first select a narcosis depth limit and determine the partial pressure of nitrogen (PPN2) while breathing air at that depth. For example, a dive to 99 feet, or four atmospheres (ATA), yields a PPN2 of 3.16 on an air dive. However, if we reduce the nitrogen content to 35 percent by replacing some of the nitrogen with helium, we can dive the resulting gas mixture to 300 feet while experiencing a PPN2 of approximately 3.16. That means our narcosis level at 297 feet (10 ATA) on that blend of trimix would be roughly equivalent to the level experienced at 99 feet on air. Similar calculations must be made for oxygen exposures at depth and during decompression.

Agencies offering trimix certifications:

> American Nitrox Divers International (ANDI) www.andihq.com

> International Association of Nitrox and Technical Divers (IANTD) www.iantd.com

> National Association of Underwater Instructors (NAUI) www.nauiww.org

> Technical Divers International (TDI) www.tdisdi.com

The weather was nippy and overcast and the water just chest-high, but a new scuba-diving pool in Paris has something Bali, Belize and other diving hotspots don’t: a terrific view of the Eiffel Tower.

To promote the sport, scuba instructors began offering free lessons Friday — with wet-suits, scuba gear and even a biodegradable towel — at the foot of the French capital’s famed landmark.

“Through the water you can see the monument. It’s magnificent,” said New Zealand tourist Adrian Carter, one of the first to try it.

He and a group of friends had planned to go up the 1,063-foot high Eiffel Tower, but opted for a dip instead.

“This is better than the Eiffel Tower,” said Carter, a 28-year-old computer programmer, his hair dripping from the 30-minute dive — his first ever.

The lessons include a safety lecture and a how-to demonstration in which instructors share tips. One first-time diver did a double-take when his guide told him to spit into his goggles to help keep them from fogging up.

The above-ground pool is under the Tower, between its four legs. It’s small, at 50 feet by 50 feet, about half the size of a basketball court. Just 4-feet deep, it’s safe for beginners and children aged 8 and older, said the event’s organizers, an umbrella group of scuba associations. To add a touch of realism, the bottom of the pool is studded with waterproof photos of fluorescent fish.

Though heated, the water temperature hovered Friday around a cool 71 degrees.

That, combined with icy winds that whistled down the Champs de Mars, a grassy promenade leading to the Tower, dissuaded many would-be divers. More people milled around the pool’s perimeter — watching the instructors as they floated on their backs staring up at the tower’s steel girders — than actually queuing up for a lesson.

This was not the first time the Eiffel Tower has become a sporting venue. Three winters ago, an ice-skating rink was installed on the lower of its three observation decks to draw Parisians to the monument that mostly attracts tourists.

Organizers of the 10-day diving event said they were angling for tourists and Parisians alike.

“We want to give as many people as possible a taste of scuba-diving,” said Gerard Puig, the pool’s manager and head of a diving company on France’s Mediterranean coast.

He said organizers also hope the diving experience will focus attention on the environmental dangers threatening the ocean.

“Once you’re underwater and face-to-face with all sorts of creatures, you can’t remain insensitive to the destruction of the sea,” he said.

Organizers expect up to 3,000 people to take the plunge before the lessons end June 10 — so long as the dismal weather improves.

Another first-time diver who took the plunge, English tourist Jonathan Doneley, said the experience was “awesome” despite the cold.

“I’m still shivering,” he said through chattering teeth. “I’m going again tomorrow.”

All tunnels leak, but this one is a sieve. For most of the last two decades, the Rondout-West Branch tunnel — 45 miles long, 13.5 feet wide, up to 1,200 feet below ground and responsible for ferrying half of New York City’s water supply from reservoirs in the Catskill Mountains — has been leaking some 20 million gallons a day. Except recently, when on some days it has lost up to 36 million gallons.

After tiptoeing around the problem for many years, and amid mounting complaints of flooded homes in the Ulster County hamlet of Wawarsing, the city’s Department of Environmental Protection has embarked on a five-year, $240 million project to prepare to fix the tunnel — which includes figuring out how to keep water flowing through New Yorkers’ faucets during the repairs. The most immediate tasks are to fix a valve at the bottom of a 700-foot shaft in Dutchess County so pumps will eventually be able to drain the tunnel, and to ensure that the tunnel does not crack or collapse while it is empty.

For this, the city has enlisted six deep-sea divers who are living for more than a month in a sealed 24-foot tubular pressurized tank complete with showers, a television and a Nerf basketball hoop, breathing air that is 97.5 percent helium and 2.5 percent oxygen, so their high-pitched squeals are all but unintelligible. They leave the tank only to transfer to a diving bell that is lowered 70 stories into the earth, where they work 12-hour shifts, with each man taking a four-hour turn hacking away at concrete to expose the valve.

The other day, one of the divers, A. W. McAfee, moved about as gracefully as anyone could in that much water, slowly fixing a monkey wrench onto a screw wedged in a block of concrete, then taking a mallet to whack it free. As Mr. McAfee did this again and again, a camera on his helmet broadcast his slow ballet and heavy breathing in near-darkness to a video feed far above in a brick building, where his bosses sat, riveted, searching for clues.

“We’re trying to piece it together to figure out the state of the tunnel,” said Jim Mueller, a deputy commissioner at the D.E.P. “This is all due diligence to stay of out of a crisis.”

Among other things, the divers have pulled out a 4,000-pound pipe elbow made of manganese bronze that was installed in 1939, finding it in “remarkably good shape,” said Nick M. Cholewka, a construction manager with the department. “If it was pulled out and all corroded, we’d be worried and we’d have to pull out more.”

New York has one of the world’s most complex water systems. Eight million residents in the city, and another one million upriver, daily consume 1.2 billion gallons that flow through a network of reservoirs and aqueducts stretching from the Delaware River watershed to the Connecticut border almost 100 miles to the southeast. The Croton system in Westchester County, which began providing water in 1842, meets about 10 percent of the city’s needs. The Catskill system, built in the first quarter of the 20th century, provides 40 percent.

The remaining 600 million gallons a day also come from the Catskill Mountains, through the Delaware Aqueduct system, which was finished in the 1960s and includes the troubled Rondout-West Branch tunnel, completed in 1944 and promised to last at least a century.

The city learned of the extensive leaks 20 years ago, but did little to fix them until 2004, when the D.E.P. hired consultants to investigate the extent of the problem. In an audit issued in 2007, the state comptroller accused city officials of foot-dragging, saying they “did not adequately monitor the extent and nature of the leaks” and had “not established an adequate plan to protect the public in the event of a sudden or imminent substantial loss of water from the Delaware Aqueduct system.”

In the last year, as Wawarsing homeowners reported mold, wells contaminated with E. coli and basements that flooded even on dry days, the city stepped up its efforts. A few in Wawarsing, a town of 13,000 that is a two-hour drive north of Manhattan, noticed cracks in foundations and sinkholes in their backyards, which they roped off to keep children away.

“No one wants to move,” said Laura Smith, a Wawarsing resident who helped form an advocacy group for the neighborhood, “but no one wants to sit here in the floods.”

At the request of the D.E.P., the United States Geological Survey is trying to determine how much of Wawarsing’s well water comes from the tunnel, but in the meantime, New York City has been providing the town’s residents with bottled water, and plans to soon send pumps and ultraviolet lights to filter the wells. Ms. Smith and others said that since last month, when the D.E.P. turned off the flow through the tunnel for the diving project, flooding has ebbed.

Yet Steven W. Lawitts, the acting commissioner of the department, said the earliest the city might empty the tunnel was 2011, because of all the preparatory work required. As the divers study the underwater pumps, city officials are investigating where, how and whether to build a bypass tunnel, and are building a filtration system to allow more water to be pumped from the Croton system.

Albert Appleton, a department commissioner in the 1990s, says the city should build another tunnel across the Hudson River connecting the four Delaware system reservoirs to the Catskill reservoirs. This tunnel, he said, would be more expensive than a smaller bypass, but would give the city more flexibility by connecting two giant water systems.

Such a tunnel would be the latest in a long list of additions to New York’s waterworks. Except during World War II and the fiscal crisis of the 1970s, “we’ve been building structures that deliver water to New York City continuously for 175 years,” said Kevin Bone, an architect who edited “Water-Works,” a 2006 book on the city’s water supply.

In the 1820s, New Yorkers used an average of 12 gallons of water a day, he said. Individual water use peaked in the 1980s, at more than 200 gallons. Through conservation, technology like low-flush toilets and repairs to the city’s leaky pipes, consumption is now about 150 gallons a day per person, said Mr. Bone, who expects it will fall further.

“We’ve always been playing catch-up until the last 20 years, when we realized the only way to get more out of the water system is to use less,” he said. “There’s no more water out there for us.”

But building water tunnels can take decades: One known as New York City Water Tunnel No. 3 was started nearly 40 years ago and will not be completed until 2015.

Just setting up the 14 tractor-trailers’ worth of gear to run the $22 million diving operation at Shaft No. 6 in Dutchess County took a month. To help the divers see better in the shaft, 1,000 gallons of sediment-filled water is pumped out every minute. Other machines filter the helium mixture that the divers breathe. Two boilers heat water that is pumped through the divers’ suits to keep them warm.

Three divers at a time climb into the steel bell, an orb that is lowered down the shaft for 20 minutes to reach the pumping equipment in the tunnel. The bell is tethered to a bundle of cables carrying air, communication lines, electricity and water. Each diver works for four hours and rests underwater for eight before returning to the tank at the surface, where 32 more employees of Global Diving and Salvage, the Seattle company running the project, pass meals, clothes and books through an air lock.

In the saturation control room, Patrick Boyd, a life-support technician, monitors the divers’ air on a panel of screens, one of which reads 2.26 percent, for the amount of oxygen. While underwater, divers often get more oxygen in their mixture to keep them alert. John Lapeyrouse, a dive supervisor who is one of the few who can understand the helium-riddled voices, one of the side effects of what is called “saturation diving,” talked to Mr. McAfee as he worked the other day.

In a tent nearby were washers, dryers and a full kitchen. The divers can request whatever food they like, including steak and fresh salads. But the air pressure in the tank dulls the taste buds, so they use a lot of Tabasco, salsa and jalapenos; bread goes flat, more pita than challah. Once the operation is complete, the divers must remain in the tank for a week to gradually wean themselves off helium.

“They lose a lot of weight because they’re burning so many calories,” said Robert Onesti, who is running the project for Global Diving. “It’s not for everybody. It’s heavy construction work, and it’s deep.”