In an underwater cave somewhere beneath Mexico, two divers, Sheck Exley and Jim Bowden, are squeezing through a narrow crevice toward a light ahead of them. The light comes from an opening, an entrance into a second cave known as Zacatón, whose abyssal depths almost defy description.

Exley and Bowden pass through the opening, then rise to the water's surface 25 feet above, emerging into a circular pool surrounded on all sides by rocky walls that reach up through the water 70 feet into the air. The stark beauty of Zacaton's cliffs, sun-baked and barren, is breathtaking but their interest lies at its bottom, 1080 feet down.

Bowden, a 52-year-old Texan whose hair and beard have long since turned an ivory white, has devoted much of his life to exploring underwater caves such as this throughout Central America and Mexico. In this part of the world, if it's underground and underwater, he's probably been there.

His partner, Exley, is universally regarded as the Michael Jordan of cave diving. An exceedingly open man to those he knows, an intensely private one to those he doesn't, Exley is an enigma in the diving world: he holds virtually every cave diving record in terms of depth and penetration, and yet, he wants no publicity for those records. At the age of 45, he's spent more time in underwater caves than most fish. His passion for them is legendary but beyond that, he has a supreme, natural ability to operate within their confines. Most divers, even highly trained ones, lose a certain percentage of their land skills once they dip beneath the water. Not Exley. His transition from one medium to the next is as effortless as it is complete and over the course of some 3000 dives, he's worked his way out of enough life-threatening situations to give Jacques Cousteau an inferiority complex.

Exley is also something of a reluctant standard bearer for the new sport of technical diving: a subculture of mission-oriented divers whose use of technology enables them to descend to ungodly depths. Rather than dreaming of dive vacations in the Caribbean, technical divers are more likely to be found searching the internet for information on diving computers, heads-up displays, rebreathers and portable recompression chambers. They use complex software to plan out their descents, carry $10,000 to $15,000 worth of equipment with them into the water, and place themselves in situations well beyond the capabilities of the average recreational diver.

At first glance, they may seem possessed of potentially fatal amounts of testosterone but in the last few years, their ranks have explored and mapped out some of the world's deepest underwater caves, descended on such previously undiveable wrecks as the Monitor and the Lusitania.

No technical diver, however, not even Exley, has ever dived to depths such as those at the bottom of Zacatón, where hydrostatic pressure zooms from the 14.7 pounds per square inch we experience at sea level, to 487 psi. Such pressure can implode even the best diving equipment as easily as someone crushes a styrofoam cup.

Its effects on the body can be just as dramatic, making the range of physiological barriers to be overcome read like an inventory of hyperbaric nightmares: decompression illness, a.k.a. the bends, oxygen toxicity, high-pressure nervous syndrome, deep-water blackout, and nitrogen narcosis -- all the result of breathing air or other gases under extreme pressure. To combat them, Exley and Bowden have spent the last year going over each detail of the dive, attempting to make it, if not safe, at least survivable.

It's April, 1994. Their dive plan is fairly straightforward. Each will drop downward into the near-vertical cave under the weight of 200 pounds of gear, maintaining visual contact the entire way with a weighted line anchored to the bottom. At these depths, the inert gases in their breathing mixtures will be absorbed rapidly into their bloodstreams. Water pressure will keep those gases compressed in small, harmless bubbles -- that is, until they begin their return to the surface. If they ascend too quickly, the decreasing water pressure will cause the gases to bubble up out of solution like a can of soda that's been shaken before being opened. The result is the bends: an excruciatingly painful, often fatal, condition.

To avoid this, after touching the bottom Exley and Bowden will rise to the surface slowly, making "decompression stops" at predetermined depths for specific periods of time, allowing the gases to come out of solution slowly. The entire process will keep them underwater for more than ten hours. The key is speed. The faster they reach the bottom and start back up again, the less gas they absorb. Toward that end, they'll descend at a rate of 100 feet per minute. To give some perspective, a commercial diver descending to 1000-plus feet would breathe through a surface-supplied hose and take upwards of 24 hours to get to his destination. Exley and Bowden will be there in 12 minutes.

Another problem is that of air supply. They can't carry a ten-hour gas supply on their backs so dozens of extra tanks, each holding a different gas mixture designed to speed decompression, are tied into each of their descent lines at the various decompression stops. Even with these tanks, however, if either diver takes more than 6 breaths per minute at any point during the descent, he'll run out of gas long before making it back to the surface.

The late morning sun is cresting over the ridge surrounding Zacaton as Exley and Bowden complete their equipment check. They're ready. One last nod to each other and under they go, heading down, down, and down some more; down through mineralized sulfur clouds held in suspension, through warm water growing ever darker. At 230 feet, they angle down around a ledge and all light fades to black. Flashlights begin to cut through the darkness.

Minutes pass. Hurtling downward silently, Bowden checks his pressure gauge at 850 feet and suddenly realizes he's used up his breathing mix much faster than expected. There's no time to figure out why. He may have enough to get down to the bottom but not enough to make it back up. To continue on would be suicide. It may already be too late. He begins inflating his BC, or buoyancy compensator -- a collapsible vest which can be inflated or deflated to control buoyancy -- but momentum and the 200 pounds of equipment he's wearing carry him deeper. He sails past the 900-foot mark. To kick or swim would be fatal at this depth since the carbon dioxide generated by the exertion would cause almost instant blackout. He can do nothing but hit his BC inflator. Hard. Finally, he reaches out to grab the line and as the 925-foot mark slips through his fingers, he comes to a stop.

Little by little, buoyancy begins lifting him back up through the darkness and, eventually, sunlight breaks in through the water. He sees Exley's descent line 25 feet away. He also sees Exley's staged decompression tanks tied into it waiting for him. But he doesn't see Exley.

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The men and women who pioneered the sport of scuba diving in the 950's were hard-assed explorers in every sense of the word. The technology was new. Its limitations, and those of the human body while using it, were largely unknown. To suss them out, they pushed them as far as they could, and then some, until, within 30 years, they could be pushed no more. Dives in excess of 300 feet were being made on air but it was physiologically impossible to go much deeper. At such depths, nitrogen, which is otherwise harmless, transforms into a mind-numbing narcotic. Oxygen, too, becomes toxic, poisoning the central nervous system and sending the body into convulsions.

The only way to go deeper is to add varying amounts of other inert gases -- helium, argon, or neon -- into the mix. The problem at the time was decompression. Since these gases are absorbed into the body at different rates than nitrogen, the standard decompression tables -- schedules of required decompression stops to allow safe absorption of the different gases and avoid the bends -- don't apply to them. And at the time, there were no alternative mixed-gas tables. That left divers with two choices: stick with air and dive no deeper than current limits, or take a chance with mixed-gas using a homemade decompression schedule. The chance was not a small one. Even minor decompression mistakes can be deadly, so breathing these mixes was akin to a game of Russian Roulette.

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In the Spring of 1988, six years before his Zacaton dive, Sheck Exley entered Naciemiento del Rio Mante, an underwater cave in Mexico, with four huge tanks containing a mixture of helium, nitrogen, and oxygen strapped to his back and sides. He had already made several mixed-gas dives over the last year but this was the first one he would make with a customized decompression schedule developed specifically for him.

We were so far off the charts in terms of depth and duration that we were going on gut feeling," remembers Bill Hamilton, who developed the dive plan for Exley. "The computer would spit out numbers but we couldn't take them at face value – they weren't based on any actual exposures. So we manipulated them, worked around them, interpolated between them -- you know,all the things you're not supposed to do -- and eventually we got a dive plan out of them."

From previous, shallower, dives, Exley already knew the hazards of Mante. Silt, which could obliterate visibility for hours if disturbed, lined its floor, while jagged edges jutted out from its sides. Strong, upward currents slowed his descent to 30 feet per minute. As he went down, the cave's shape began to hourglass; growing wider, then narrower, then wider again, all the while maintaining its nearly vertical drop. His goal was to dive beyond 700 feet, deeper than anyone had ever been before
with scuba.

Twenty minutes later, Exley passed below the 700-foot mark, even as the narcotic effects of the nitrogen crowded in on his senses. Suddenly, an explosion behind his head almost knocked him unconscious. He steadied himself against the wall of the cave and tried, without luck, to locate the problem.

He checked his depth gauge. He was now 780 feet deep. Whatever the explosion was, it was time to head back. He rose up quickly into shallow water (shallow being a relative term) and began his decompression, breathing from tanks he had staged along the way, each of which contained the precise gas mixture called for by Hamilton's decompression schedule.

Timing was essential. To linger too long, or not long enough, at any one stop would be an open invitation to the bends so he kept a close eye on all three of his watches. All three watches? That was a habit he picked up the year before when he lost the one watch he was wearing mid-dive and was forced to count off his decompression stops in his head. One-one-thousand, two-one-thousand, three-one-thousand. Luckily, he only had to do
it for a couple of hours.

And the explosion? A plexiglass case housing the battery that powered his light had imploded. Just couldn't take the pressure.

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"Would you venture out into the unknown out of greed? Greed works only in the world of ordinary affairs. To venture into the terrible loneliness one must have something greater than greed. Love, one needs love for life, for intrigue, for mystery. One needs unquenching curiosity and guts galore."

With that somewhat purple prose from Carlos Castenada's The FireFrom Within, Mike Menduno, a former marketing executive going through a mid-life crisis, launched aquaCorps, a dive magazine with an attitude, in 1990. Until that moment, in the 40-plus years since Jacques Cousteau had introduced the world to the scuba tank, diving had steadily evolved from the province of exploration to that of a tame, organized sport, generating millions of dollars on the sale of equipment and travel vacations. New diving rules, established by training agencies and bolstered by dive magazines, the sport's new power structure, were set in stone in the 1960's: never dive deeper than 130 feet, never dive alone, never get into decompression situations.

The sport of rugged individualism, of Lloyd Bridges on the old TV show Sea Hunt, soon had an accident rate lower than that of bowling. All vestiges of the bad old days, of explorers diving by the seat of their wet suits, were stricken from the record and people just entering the sport had no idea that things had ever done any other way.

aquaCorps took a different approach. Standard fare included deep diving, mixed gases, improved decompression methods, treatment strategies for the bends, and fatality reporting -- topics no other magazine had touched in 25 years. In one fell swoop, all the divers your mother warned you about had a pulpit from which to preach. "People who were already into deep diving and mixed-gases grabbed it up," says Menduno. "But the powers that be, well, they were a little bit cautious."

A little bit apoplectic would be a better description. Angry, almost hysterical, editorials filled the pages of other dive magazines. Manufacturers of some types of mixed-gas equipment were banned from the industry's annual trade conference. Under the threat of legal action, they were then reinstated.

By the time the second issue of aquaCorps hit the stands, Menduno had christened the unnamed sport "technical diving." Even as he did, it became clear that its demands were far less forgiving than those of its gentler predecessor. In the summer of 1992, eight divers in the US died during deep or mix-gas dives. There were more deaths in Europe.

Nevertheless, the tide had turned. The allure of exploring virgin and near-virgin caves, wrecks and reefs, all the while pushing personal limits to the edge, testing them, extending them, was too powerful to be ignored. Training agencies for technical diving were being established. aquaCorps had launched tek, an annual conference that was drawing divers in from around the world. Even the more mainstream dive magazines were touching on technical diving topics previously taboo. And as more deep, mixed-gas dives were conducted, data on their limits were accumulated, collated, and disseminated, eliminating some of the old dangers, even as newer ones were tempted.

Scuba had finally come full circle: from the first explorers who strapped on archaic tanks back in the 1950's, to technical divers who now strapped on underwater computers.

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By 1993, the growth of technical diving had vaulted Sheck Exley into legendary status among his peers. "He was walking on the dark side of the moon compared to everyone else," says Bret Gilliam, a longtime friend and CEO of Technical Diving International. "When he made his original Mante dive, not only did he go deeper than anyone had ever been before, even his first decompression stop was deeper than anyone had ever been." Among technical divers, it was becoming clear that he was The Man.

That summer, he used a decompression schedule developed with his own software to dive a water-filled sinkhole in South Africa named Bushmansgat. Squeezing through a three-foot crack to enter it, he picked up speed and descended 200 feet into the world's largest underwater cavern. Even to Exley, its size was overwhelming.

He continued down, at a faster rate than ever before. But at 700 feet, his body lodged a severe protest. Hundreds of small concentric circles, each with a sparkling dot in its center, filled his vision. His body began itching and stinging. It was high-pressure nervous syndrome, or HPNS, a little-understood phenomenon brought on by severe, rapid compression, that inhibits the function of the brain, causing neural circuits to run wild. Exley had never experienced it before.

He hovered at 750 feet, considering his options: abort the dive or continue down. Decision made. He started down again, more slowly this time, though the change did little good. By the time the cave bottomed out at 863 feet, his entire body was shaking uncontrollably.

Exley ignored it. Even without full vision, his flashlight cut through the darkness and he could make out what appeared to be a
lunar landscape: a uniformly flat bottom sloping away from him, covered with small rocks, buried in nearly a foot of black sediment. No one had ever stood where he was standing. He couldn't stay long since every minute of extra time he spent on the bottom would have to be paid for with extra hours of decompression. After a good look around, he inflated his BC and began to rise. He was more than half way up before the HPNS subsided.

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One hundred feet beneath the water's surface at Zacaton, Jim Bowden hovers at his decompression stop. Above him, the sun has set below the ridge surrounding the cave. Night has fallen and a dry wind is blowing across the Mexican plain. A stand-by diver moves in close to him and begins signalling to him. As she does, the silent motions of her fingers weave a message that he already knows in his heart but has tried not to believe. Sheck Exley, the man who had rescued countless other divers from near-fatal situations, has not come up. Sheck Exley is dead.

Bowden continues to hover. He can't head back down for Exley; he doesn't have the gas supply. He can't surface; to do so without finishing his decompression would be certain death. He can only wait, locked into the silence of a diver underwater, suspended in decompression, mourning his friend. "That was a horrible moment," remembers Bowden. "We knew the risks we were taking but even still, it was hard to imagine Sheck not surviving the dive -- not surviving any dive."

Within hours, reports of Exley's death go out over news wires. aquaCorp's office is swamped with e-mail. rec.scuba on the internet is also filling up fast, as are the scuba bulletin boards of all of the major on-line providers. The thought of Exley not returning from a cave dive, even one to 1080 feet, is as incomprehensible as that of a young Muhammad Ali dying in the ring from a punch. "To find out that he was mortal came as quite a shock to a lot of people," says Bret Gilliam.

No one expects that Exley's body will be recovered from the depths of Zacaton -- who could go down to retrieve it -- but several days later, his descent line, heavy with his unused decompression tanks, is winched up and his body, wrapped into it, is recovered. An autopsy is inconclusive, and with no eyewitnesses, it's impossible to know with any certainty what truly happened to him.

The most likely scenario is that, like Bowden, for unknown reasons he used up his gas much faster than expected. Unlike Bowden, however, he mustn't have realized it until too late. At that point, without enough gas to inflate his BC and rise, he probably wrapped himself into the line to stop his descent and give himself time to think. At that moment he was most likely experiencing flashes of narcosis as well as another episode of HPNS. When he ran out of gas mixed for that depth, he switched to another one that could only be breathed safely much closer to the surface and, at that point, he suffered an oxygen convulsion, lost consciousness and drowned.

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At the '95 tek Conference in San Francisco, Exley's ex-wife and longtime dive partner accepts a posthumous award in his honor. Other than that, there's little mention of his death. Maybe the topic has been talked out. Maybe it's time just refusing to stand still.

The big draw, as at all tek conferences, are the emerging technologies that promise to push dive capabilities of the near future way beyond current limits: communication systems that will allow speech underwater; dive computers with magnetic keypads for the input of data such as navigational coordinates and underwater e-mail; software programs that will calculate everything from mixed-gas decompression schedules to oxygen tolerance units.

More than 60 manufacturers are on hand to showcase their wares. Some, who have manufactured exclusively for commercial deep-sea divers for the last few decades, try now to break the recreational line for the first time. Others are former Navy suppliers looking for greener pastures since the end of the Cold War. The one thing they have in common is technology that will enable divers to go deeper, longer, farther.

"I think they're finally starting to realize that this is no longer just a niche segment of the scuba market," says Tom Mount, president of the International Association of Nitrox and Technical Divers. Indeed. More than 2000 people attend the '95 tek conference, and there are an estimated 8000 more technical divers in the US. The majority are educated, professional, and have disposable income to spare. More importantly, their numbers are growing faster than any other segment of the scuba community. Everywhere. 1995 will see the first Eurotek conference, with Asiatek not far behind.

As for Bowden, he's still planning to reach the bottom of Zacatón, just like any mountaineer determined to see the top of Everest. "You know, underwater caves are truly the last frontier on Earth," he says. "They're the only places left that you can go to that haven't been walked on, sonar-sounded or satellite-photographed. And that's what drives me -- that's the thrill. Now, you can call it ego or try to cheapen that feeling in any number of ways but that's what it comes down to. I'm going to keep on exploring. And so will others."

copyright 1994 Jerry Shine