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Northern Mexico.

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The sixth of April, nineteen ninety-four. Late morning.

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Two men slip into a round pool of warm water, ringed by rock walls that rise about seventy feet above it.

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Each of them is carrying about two hundred pounds of equipment.

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Each of them has his own weighted line, running straight down into the dark.

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They are going to drop at a hundred feet a minute.

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The bottom, by the soundings, is more than a thousand feet below them.

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One of them is the most famous cave diver in the world.

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The other is his partner on this dive, and his friend.

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Only one of them will come back up.

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This is Caving Stories: Cave Diving. True stories from flooded caves, told from the record.

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Today, a sinkhole called Zacatón, and the last dive of Sheck Exley.

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Sheck Exley grew up in Jacksonville, Florida, a state full of rivers and sinkholes, and full of water in the rock beneath them.

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He took his first scuba course at sixteen.

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On his very first dive, he wandered away from the group and found a small opening in the limestone.

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"I would have given anything," he wrote later, "to have had a light so I could see where the tunnel went from there."

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His instructor chased him back to the boat. He was about to scold him, then saw the look on his face.

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"You're hooked," he told him.

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In those years there were no rules for diving in caves. The few people doing it learned by surviving their mistakes.

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Early on, Exley and a friend got tangled in their own guide line at a sinkhole in Florida.

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So later that same day, at a cave called Peacock Springs, he dived without one.

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He went about four hundred feet into the cave, alone, with a partly empty tank and a flashlight whose battery was dying.

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When he turned around, the branching passages all looked the same.

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He swam to what he thought was the way out, looked up for the open water, and found solid rock.

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His air began to run out. He pulled his reserve lever, which gave him about five more minutes.

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He found a crack in the ceiling that he called impossibly tiny, and squeezed through it with his arms pinned to his sides.

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He came out by a different route from the one he had gone in by.

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He never went into a cave without a guide line again.

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That was not the worst of it.

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Exley had a younger brother, Edward. One day Exley watched him take a few breaths at the surface, swim down, and disappear behind a ledge.

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A minute later he came back up at a strange angle. At the surface he kept on swimming, and then he slowly began to sink.

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Exley and the others gave him CPR for an hour, and got his heart and lungs going again. He never regained consciousness.

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"My only brother, Edward, was dead," Exley wrote. "I was the one who had to make the call to my parents."

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What he did with the rest of his life was, in large part, about safety.

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He was the first person to log a thousand cave dives, by the age of twenty-three. Over his life he made more than four thousand.

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He paid for it by teaching mathematics at a high school in Florida.

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He refused to teach his students to dive.

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In nineteen seventy-nine he published a short book, Basic Cave Diving: A Blueprint for Survival.

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It set out the rules that cave divers still learn: training, a continuous guide line, a depth limit, a third of your air to go in and a third to come out, and three lights.

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His biographer in the Journal of Diving History put it this way: he changed cave diving from a renegade, deadly activity into a highly disciplined sport.

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And then he went deep.

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In Mexico, in a spring called the Nacimiento del Río Mante, he dived to five hundred and twenty feet in April of nineteen eighty-seven, an American record.

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On the way up he lost his watch at his two-hundred-sixty-foot stop, and counted off the rest of his decompression on his fingers until Mary Ellen Eckhoff brought him a replacement.

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In June he took the line to six hundred and sixty feet. In April of nineteen eighty-eight, to seven hundred and eighty.

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By one account, that was deeper than any scuba diver had been before.

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There were no tables for dives like these. A physiologist, Bill Hamilton, built his decompression schedule by hand.

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"We were so far off the charts," Hamilton said, "that we were going on gut feeling."

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In March of nineteen eighty-nine, Exley reached eight hundred and eighty-one feet, breathing a mixture of oxygen, nitrogen and helium.

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The climb back took fourteen hours of decompression.

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At these depths the body starts to behave strangely. On one deep dive, Exley described a shaking that grew uncontrollable by the time he reached the bottom, and went away only when he came back up to about three hundred feet.

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In nineteen ninety-three, at Bushmansgat in South Africa, he dived to eight hundred and sixty-three feet, where that cave ran out.

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He read paperback novels during his long decompression stops.

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Zacatón is in Aldama, in the state of Tamaulipas, in north-eastern Mexico.

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It is a sinkhole full of warm water. Islands of grass float on its surface and drift with the wind. In Spanish, that grass, zacate, gave the place its name.

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It had been sounded to more than a thousand feet.

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One of the people who knew it best was Jim Bowden, an explorer from Texas who had spent much of his life in the flooded caves of Mexico and Central America.

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The year before, in nineteen ninety-three, Dr. Ann Kristovich had dived there to five hundred and fifty-four feet, a women's world record.

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Exley and Bowden would go together, and try for the bottom.

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The plan was simple to say.

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Each man on his own line, so that they could not foul each other.

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Down at a hundred feet a minute: twelve minutes to the bottom.

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Tied to each line, on the way up, were dozens of extra tanks, each with a different gas mixture, for a different stage of the climb back.

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The whole dive would keep them underwater for more than ten hours.

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The margin was thin. By one account, if either man took more than six breaths a minute at any point on the way down, he would not have the gas to get back.

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On the surface that day, with them, were Mary Ellen Eckhoff, Ann Kristovich, Karen Hohle, who was Bowden's wife, and Marcos Gary.

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They went under together.

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Through the warm water, through clouds of sulfur hanging in it, down past a ledge at about two hundred and thirty feet, where the last of the daylight goes.

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From there on, only their lights.

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At eight hundred and fifty feet, Jim Bowden looked at his pressure gauge, and saw that he had used his gas much faster than he expected.

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He might have enough to reach the bottom. He would not have enough to come back.

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He started to inflate his buoyancy jacket.

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But the weight of all his equipment, and his own momentum, kept carrying him down. Past nine hundred feet.

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At that depth he could not kick his way up. The effort would make carbon dioxide build up in his body, fast enough to make him black out.

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So he put more air into the jacket.

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He reached out, and caught the line.

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He stopped at the nine-hundred-and-twenty-five-foot mark.

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It was recorded as a world depth record.

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Slowly, the jacket lifted him back up into the dark, and eventually into water with sunlight in it.

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Twenty-five feet away he could see Exley's line, with Exley's decompression tanks tied along it, waiting for him.

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He could not see Exley.

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Bowden had hours of decompression ahead of him.

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He was hanging at his stop at a hundred feet when the sun went down behind the rim of the sinkhole.

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A support diver came down to him, and began to make signs with her hands.

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He already knew what she was telling him.

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Sheck Exley had not come up.

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He could not go back down for him. He did not have the gas.

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He could not go up. Without finishing his decompression, he would die.

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He could only wait in the water, for hours, and think about his friend.

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"That was a horrible moment," he said later.

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No one expected to see Exley again. A body at that depth is not recovered.

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Several days later, the team hauled up his descent line, heavy with the decompression tanks he had never used.

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Exley's body came up with it.

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He was wrapped in the line.

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According to the published record, the line was wrapped around the valves of his tanks deliberately.

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His dive computer recorded how deep he had gone. The published figures differ: eight hundred and seventy-nine feet in one account, nine hundred and six in another.

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Either way, the bottom of Zacatón was still far below him.

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The autopsy did not settle the cause.

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There were no witnesses.

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So everything that is said about the last minutes of Sheck Exley is a reconstruction, and the sources say so.

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His biographer writes that it is believed he suffered an attack of the shaking he had felt before, the high pressure nervous syndrome, ran low on gas, and drowned.

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Whether he wrapped himself to the line because he knew he was in trouble, or while he tried to solve a problem, is unknown.

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Almost everyone agreed that he was unlikely to have tangled himself by accident.

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The account reprinted by IUCRR goes further. It suggests that, like Bowden, he used his gas faster than expected, did not notice in time, tied himself to the line to stop his fall, and then switched to a gas meant for much shallower water.

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That, too, is a theory.

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Bowden's dive set a world record.

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Zacatón's bottom was later measured by a robot.

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Exley had written about this kind of diving years before.

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"On deep dives," he wrote, "you move into the physiological and psychological unknown, where all the components of the air that you are now breathing become poisonous, where the normal problems of diving in caves are magnified a hundredfold."

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Bret Gilliam said what many people felt.

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"To find out that he was mortal came as quite a shock to a lot of people."

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The rules he wrote down are still the basis of how cave diving is taught.

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The sources for every fact in this episode are in the show notes.

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This has been Caving Stories: Cave Diving.

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Thank you for listening.