They came to their second restriction twenty-five minutes into the dive. He couldn’t see how far the narrow shaft went before opening up again, and he didn’t want to get stuck in the middle of it with the scooter, so he tied it off on the guideline. He gave Elsa an okay hand signal, she okayed back, and he entered the restriction using what was known as the bent-knee kick. Propulsion was limited, but the technique worked well in cramped areas. Most importantly, it minimized the disruption of silt on the cave floor, which could turn gin-clear water into a soupy brown mess, the last thing a diver wanted when there was no direct access to the surface.

The initial twenty feet into the restriction went smoothly, but then the shaft flattened to a horizonal wedge, forcing Marty to wiggle through, scraping his chest and tanks on the rock and stirring up the fine silt that had gathered there over the years. His visibility deteriorated to two meters. As he fumbled deeper along the squeeze, it dropped to zero. He couldn’t see his hands when he held them before his face, forcing him to feel his way forward.

Quiet panic nibbled at his insides, and he told himself to remain calm. Panic led to fear, and fear shut down rational thought.

After another twenty or twenty-five feet of slow progress through the blackout, the passage corkscrewed, becoming more vertical than horizontal. Marty twisted his body to compensate—and at some point realized he no longer knew up from down.

The panic returned, louder and hungrier. Marty’s breathing became more rapid. Thoughts of hypoxia and carbon dioxide poisoning snowballed the panic, causing the mildly stressful situation to spiral into full-blown anxiety. He began kicking faster and clawing at the rock to escape the claustrophobic space. His throat felt clogged with coppery pennies, and his hand went to his respirator, instinct shouting at him to yank it free. He nearly followed the fatal advice, but some higher-level cognitive process stopped him.

Stop—think—breathe.

He forced himself to stop flailing.

Stop—think—breathe.

He forced himself to take deep, even breaths.

Stop…think…breathe…

His heartbeat slowed. The heaviness in his chest subsided. The sense of impending doom faded.

And before he knew it, he could see his light again, and his elbows and knees were no longer slapping against rock.

He was out!

The relief was immeasurable, yet he remained completely disoriented. Only when he noted which direction his bubbles were traveling did he once again know up from down. He quickly finned back the way he had come, concerned that Elsa might have panicked too. He was steeling himself to swim back into the cloud of silt when she emerged from it. Behind her mask, her eyes were intense yet calm.

He gave her the okay sign; she okayed back.

Jesus H. Christ, he thought, realizing just how close he had come to killing them both.

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