Tiptoeing along the horizontal conning tower, Nash lay down beside Boerman and craned his head over the parapet. A set of fixed pressure-proof binoculars stared back at them. It was easy to imagine the Japanese captain scanning the horizon for enemy ships, unaware the greater threat lay beneath his feet. Stumps of periscopes, and the remains of radar cones, also projected from the deck. Boerman then performed another outrageous feat of free climbing, trying to get the hatch to open while hanging upside down.

When at last he conceded, puffing and panting, they went back along the conning tower and continued the terrifying climb down through the rusting steel framework. The rear section, which had spent more time underwater, was severely corroded. At times, they literally had to kick footholds in the rust until, at last, they were able to stand without danger of falling on the flattened, bulging stern.

Nash eagerly surveyed the carpet of young tree ferns growing strongly in the shallow silt of the sinkhole floor. There were no older plants except for plucky dwarf specimens clinging to the walls above, suggesting inundation was indeed a regular event. There were several promising overhangs in shadow, suggesting caves and passages, but it was the small stream bubbling its way down to the low point which pointed to the location of the main exit passage he was expecting.

‘After you, Nash.’

With mounting excitement, Nash clambered down over the split-open pressure hulls and mangled propellers to reach solid ground.

It was only now they were able to appreciate the behemoth above their heads. Boerman whistled, for it was truly an astonishing sight. Foreshortened, the gigantic submarine looked like a tinplate toy lost down a drainpipe after a storm. Nash had to remind himself that the chaotic tangle of twigs wedged tightly around its bow were actually massive trees. It looked disturbingly like a giant set of Jenga. If one log went, several hundred tonnes of timber were poised to follow.

A gaping crack in the submarine’s hull, forty-five metres above their heads, drew their eyes. Floor plating was spewing out, and severed pipes looked ready to fall. Chunks of broken metal dotting the floor of the sinkhole suggested this was a regular occurrence.

‘Well, we have a way in,’ said Boerman cheerfully.

While the big man took shots with the GoPro, Nash surreptitiously looked around for a likely weapon. If he could escape through the exit passage, double back and take Sura by surprise, he would have a pistol and a pilot. Frustratingly, the fragments of rusting steel were either too big or insubstantial. It would have to be a stone.

He looked up to find Boerman assessing him with a shrewd gaze which suggested he knew full well what Nash was thinking.

‘OK, Mister Expert, let’s check out the rest of this bleddy hole. I’ll be right behind you.’