It took Nash several moments to comprehend what had happened. He was pinned, upside down, with Boerman lying just below him. Like enemy soldiers in the same trench, their eyes met in a temporary truce. Above their heads was instant death. No bullets or shells hurtling past, just a subterranean torrent, ready to snatch them if they left this protective lee of rock. It was as if they were stuck in the throat of a sea monster, and beyond, cruelly framed by its jagged mouth, was the pale, inviting green of the cavernous lake.

Somehow, they had to get back there.

Nash made a climbing motion with his hands. When Boerman shook his head, Nash angrily drew a cutting motion across his throat. It was a simple equation: climb or you’re dead.

Twisting himself around, Nash braced against the ledge and extended forwards, flattening himself against the rock. The vortex was buffeting his rebreather, transmitting force to his shaking knees. With his legs fully extended, he reached tentatively out with his right hand and found a handhold on the rock. Keeping his left leg flattened, he drew it up almost ninety degrees, until he located a ridge below his heel. It was the moment of truth. If he couldn’t make the next step, all hope was lost.

The effort was like climbing a wall with a person strapped to your back. But fear lent Nash a desperate strength, and he fought his way upwards to secure the next handhold. With each metre gained, the mouth of the passage widened and the force of the water diminished. By the time he reached the mouth, he was able to see Boerman right behind him, and climbed around the lip to reach the haven of still water.

When he checked his watch, Nash felt even colder. Thanks to the Afrikaner’s stupidity, their maximum depth had touched fifty metres, and their run time was almost half an hour. Now they were going to have to decompress for at least a quarter of an hour.

Nash was borderline hypothermic, but it was better than getting bent. With every added metre of depth, nitrogen became exponentially more dangerous under pressure. Coming up too fast was like shaking a bottle of Coke. Millions of tiny bubbles, tearing their way out of capillaries, was not only agonising; it could lead to permanent paralysis or death.