They were traversing the rear wall when a powerful downdraught of frigid pressure struck like an invisible hand and shunted them below 155 metres. Nash circled around to find the cause was a jagged fissure, no larger than a single-car garage, which marked the inlet of the Hoosenbeck. The immense power could only be explained by a charged passage, something like the intake for a hydroelectric generator on a giant dam. Nash realised the water column must extend for hundreds of metres above their heads, all the way to the top of the gorge and the mountain catchment. A secret vertical river, which, quite likely, was inexplorable.

Nash kept nervously rechecking his computer as he guided them towards the front of the cave. Seven minutes. Already, that was two and half hours of deco, and they had seen nothing for the whole 200-metre stretch.

Nine and a half minutes.

Traversing the front wall of the cave, all they found were giant heaps of boulders.

Eleven minutes.

The front left quadrant. As the distance unravelled, Nash knew that unless the submarine had sunk vertically, or had broken into two halves, it simply wasn’t there, and thus his usefulness was about to come to an end.

Boerman was anxiously swinging his head from side to side. At any moment, the big man was going to come to the same inescapable conclusion. Against three, Nash knew he stood no chance, but against two, with the shadows of the cave at his disposal, he might yet find a way to escape and save Mia and Frank.

Boerman was far too strong for direct physical confrontation. Nash considered cutting the big man’s rebreather hoses, but entanglement at this depth would probably kill them both. And should Boerman engage his buoyancy control device, his body would surface.

The back of the Afrikaner’s unprotected head was the solution. Abruptly pulling into a steep climb, Nash gained height before angling back down. At full speed, the nose of his scooter would crack a human skull like a soft-boiled egg.

Nash was almost at maximum acceleration when something on the floor of the lake came into view – a cluster of streamlined tubes. At the last second, he aborted, sweeping around in a tight circle to rejoin the excited Boerman, who looked around and waved a triumphant fist.

It was a seaplane, lying upside down, and in such good condition that it might have been sunk last week. Twin floats stared mutely at the surface, and the red circles underneath each wing were unmistakably Japanese. Nash recalled they had developed a gigantic submarine which could carry several aircraft. How this one had got in here was a mystery. He could only assume someone had flown inside the mouth of the cave – an astonishing feat.