The haphazard affair swayed underfoot as Nash tentatively detached from the cable. Typical Boerman! And it was a sickening straight drop of fifty metres to the floor of the sinkhole, with one lousy guide rope to hang on to. Nash hadn’t planned on looking down, but halfway across, he couldn’t resist checking the crescent pool. Its area had already increased by a third as it crept across the sinkhole floor, and Nash realised the lonely beach he had spent the night on must now be completely flooded. Had he stayed another hour . . . he’d be dead.

The gantry bowed and creaked ominously as Boerman joined him. Taking a secure hold of Nash’s harness, he bared his big teeth in a grin.

‘I go, you go. Now, tell me what is happening down there.’

‘It’s raining in the mountains and the run-off is entering the Hoosenbeck Cavern and finding its way into the sinkhole from below.’

‘Ja, ja, Nash, I know that, but how?’

‘This sinkhole must be on a side branch of the underground river. As its volume increases, it’s backing up and filling the sinkhole from below.’

Boerman’s frown betrayed his incomprehension, and Nash used his hands to demonstrate.

‘The underground river runs from the exit passage you almost drowned us in, remember? To the resurgence, two kilometres downstream.’

He did not share with Boerman what was going to happen once the subterranean discharge reached full capacity. Then, the Hoosenbeck Cavern would quickly overflow and pour down the old river bed; a torrent of white water would plummet into the sinkhole, and that huge passage he had free-dived would become a raging rocky drainpipe, at least a kilometre long! Just the thought of it made him shudder. Thank God he was never going back in there again.

‘Enough chit-chat.’ Boerman propelled him towards the submarine. ‘Now we work.’

Bracing himself on the swaying plywood, Nash stared into the cavernous belly of the I-403. Inside the twin fused inner pressure hulls, dark passageways ran near-vertically towards the bow-like elevator shafts. There was another deck above this one, but from this angle he could not see far inside. Looking aft, he saw the hull had split along a bulkhead dividing the offset control room on the port side. The huge double-height space incorporated the base of the conning tower. A periscope dangled down, its bronze handgrips seemingly awaiting a captain’s guiding hand. Porcelain gauges gleamed whitely through their rusting cases. He could see levers and valves, the frames of rotted seats and an illegible sign on the wall.

‘You first, Nash. We’ll start with the bow.’

Nash craned his head up. The long narrow tube was unnerving. God only knew what it would be like underwater.

The big man gave him a shove. ‘Move your yellow ass, man.’