Gracia was trapped in that underwater cave for 60 hours, not knowing whether his diving partner, who he had sent to get help, even made it out of the maze below the Spanish island of Mallorca.

Gracia’s ordeal began as a routine expedition. As a geology teacher, Gracia made a habit of spending his weekends mapping the island’s underwater cave routes. He was a skilled cave diver and had done everything right that day. But that didn’t prevent the random accident that turned the adventure into a nightmare.

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One of the basic tenets of cave diving is to always use a guideline. As divers swim deeper into a cave, they leave a line behind so they know the exact route out. A rope also allows divers to tow themselves out without necessarily needing to see, so that in case their lights burn out or there’s low visibility, they can still safely ascend.

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Gracia, 54, and his diving partner, Guillem Mascaró, left a rope that day, according to the BBC. But something ended up severing it, leaving them lost and with a dwindling amount of air in their oxygen tanks.

“We can only guess some rocks had fallen on it,” Gracia said, noting that the pair spent about an hour searching for the rope to no avail.

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Their only saving grace was their knowledge of an air pocket in the cave system. That would allow the pair to regroup and formulate a plan without using up all their supplemental oxygen.

And so, with only enough oxygen left to allow one of them to leave, they decided Mascaró would go in search of help. The two settled on a route that had been previously mapped, but it was long and risky. And because of the frantic searching for the guide rope earlier, there was a significant amount of silt kicked up into the water, making it hard to see.

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“It was like diving in a bowl of cacao,” Mascaro later told Diaro de Mallorca.

After Mascaro took off, Gracia’s next challenge was to maintain his sanity while breathing the cave air, which has a much higher proportion of carbon dioxide compared to surface air. According to the National Institutes of Health, carbon dioxide acts as an asphyxiant, meaning the more you breath in, the less it allows your body to absorb the oxygen in the air.

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“Exposure can also cause dizziness, headache, sweating, fatigue, numbness and tingling of extremities, memory loss, nausea, vomiting, depression, confusion, skin and eye burns and ringing in the ears,” per NIH documents.

Gracia, who suspects he was breathing air saturated with up to 5 percent carbon dioxide compared to surface air, which averages just .04 percent of the gas, experienced some of these symptoms, as well as hallucinations, as he struggled to survive.

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He told Diario de Mallorca in April that he spent most of this time in complete darkness, using his flashlight only sporadically to save its batteries. He needed the light to be able to scoot down the rock where he sat in the 80-by-20-by-12-meter space to retrieve fresh drinking water that he skimmed from the top of the cave pool’s surface. He had trouble sleeping the longer he was down there, preoccupied with thoughts of his two children, a 15-year-old son and 9-year-old daughter, as well as the possibility of his own death.

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“When [rescue workers] find my corpse, they won’t even be able to get it out of here,” he recalled thinking at the time to Spain’s El Pais (via Diario de Mallorca). “The pathways around this cave are very convoluted and it will be impossible to handle an inert body. . . . Literally, I was buried alive.”

According to the group International Underwater Cave Rescue and Recovery, more than 475 divers have died on cave expeditions worldwide between since 1950. The exact number is unknown, however, because not all fatalities are reported. Fortunately, in this case, Gracia was not added to that tally as help eventually arrived.

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“I thought it was another hallucination,” he told the BBC when he spotted a light underwater. “But then I realized it was real and I saw a helmet emerging.”

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Gracia was supplied with more oxygen and led out of the cave, where he met Mascaro, who made it out, too. Suffering from symptoms of hypothermia, Gracia spent a night in the hospital, where he was also give supplemental oxygen to recover from the damage done by breathing so much carbon dioxide.

Gracia didn’t remain on land for long, however. When he recovered, he decided to go back into the water, and, in fact, it sounds like he never thought twice about ditching his dangerous hobby.