Nash nodded. ‘And then they waited for the water to rise like a lock. That is how Heider was able to get the submarine into the cave.’

Boerman looked aggravated. ‘So where did all the water go, huh?’

‘We’re almost there.’

Nash began walking again. Sura came up alongside him.

‘Are you saying the dam collapsed and swept the submarine away?’

‘No, the dam was eroded later over time.’

‘Then I don’t understand.’

‘Cast your mind back to seventy-odd years ago, when this gorge was flooded for more than a kilometre. Can you imagine how much all of that weighed?’ He did a quick calculation. ‘Let’s call it forty million tonnes, bearing down on this very rock.’

‘Seems solid enough to me,’ said Sura.

‘Limestone is actually riddled with subterranean pockets. Existing voids would have expanded as the water forced its way in through cracks, and steadily dissolved the weakest stone. Years passed, until a critical mass was reached, when the weight of all that water became too much for a large cavity to support.’

Ahead, on his left, Nash saw the vegetation thinning out. He had spotted it on the way in and, with the lure of the cave, hadn’t paid it much attention.

‘And then?’ Boerman demanded. ‘Come on, man, finish it.’

Nash came to a halt and turned around. ‘And then, in one catastrophic event, the roof of the cavity collapsed. As millions of tonnes of water poured into the void, the level of the dam dropped suddenly. The steel hawser tethering the submarine snapped like string, and it was swept out of the cave, down the gorge . . . into this!’

Boerman and Sura stared in amazement as Nash pointed through the trees. Adjacent to the main river channel was a sinkhole. Jammed with huge logs, shrouded in hanging vines, it was almost completely camouflaged. Making their way cautiously to the crumbling edge, they peered into the gloomy depths and Sura gave a small cry. For there, emerging vertically like some mythical sea monster from the depths, was the rusting bow of an enormous submarine.

With eight torpedo doors and a hull three storeys high, the huge scale reminded Nash of nuclear leviathans he’d seen at Pearl Harbor, but the bizarreness of its last-century oriental curves in this exotic jungle location made it appear like a relic from a forgotten civilisation.

Boerman took Sura’s hand and kissed it several times.

‘You did it, liefling. I knew you were right all along!’

Sura was blushing like a bride. ‘Of course, we must thank Mr Nash, too.’

Was the compliment his eulogy?

With his usefulness at an end, Nash braced himself for a surprise push in the back.