Already, Nash’s fingers were wooden in the intense cold. Laboriously, he and Boerman finned back to the mouth, then returned to the far end again, each pass ticking off a 20-metre-wide swathe. After twelve minutes of immersion, Nash was shivering. Boerman was agitated and aggressively gesticulating towards the bottom, but Nash shook his head. Risking a long decompression for a non-existent submarine was insane. At least this way they could determine if the whole floor was beyond eighty metres and live to tell the tale.

At the end of the fourth pass, in the middle line of the cavern roughly forty metres below the cave mouth, Nash spotted what he believed to be the exit passage for the Hoosenbeck. The hole was a ragged trapezoid some fifteen metres across, which steadily narrowed as it drilled into the rock. At a guess, fifty to sixty cubic metres of water per second had to be flowing through this funnel – a force incomprehensible because it was invisible. Once, it would have tempted Nash to explore the hydrology of the system. Now, he just wanted to get the hell out of there.

Reluctantly, he turned to warn Boerman of the danger and was shocked to find the big man wasn’t there. Thinking he must have baled, Nash looked up. By the time he looked down again, the Afrikaner was six metres below, blithely swimming into the hidden vortex.

With Sura’s threats ringing in his ears, Nash went after him. Every extra metre descended meant more decompression, more time in the frigid cold. His calf muscles were tightening – a precursor to cramp. Fighting it off, he kicked harder. He was just two metres behind Boerman when the Afrikaner stopped swimming and tentatively stretched out his hands. Nash knew it was the periphery of the vortex. Then he felt its clutches, too.

Adrenaline pumped energy into his exhausted muscles, as he kicked savagely for the surface. Suddenly, he felt the big man grab his ankle.

They grappled in a desperate tandem, and then Boerman got hold of his weight belt. One look at the Afrikaner’s terrorised eyes told Nash the man would never let go, so he stopped fighting and encouraged Boerman to swim with him. Doggedly, they swam up as the gaping mouth yawned. But, for every metre gained, the vortex gobbled two.

Too late, Nash felt the acceleration in the pit of his stomach.

Bang!

He grunted in shock as his rebreather slammed against the rock of the exit passage. They tumbled together for another three or four metres, until a ledge brought them up short with a bone-jarring thud.