Ascending at nine metres a minute, they reached Hartono’s line and stopped for just two minutes at a depth of 108 metres. It was the start of a complex five-hour-plus decompression sequence, to purge themselves of the dissolved nitrogen in their tissues.

Long deco stops were usually best treated as meditative spaces, but here every minute ticked past interminably as Nash tried to figure out his next move. A seaplane was all well and good, but finding the submarine that had launched it? It could be anywhere, including a thousand kilometres of muddy Sepik river bottom.

At long last, they surfaced next to Hartono, and cruised back across to the ledge where the midday sun streamed prettily into the cave mouth. Hartono helped lift the scooters out and hook them up to the generator to recharge, while Boerman went into a huddle with Sura.

Over a hot, sweet coffee, which stung his swollen mouth, Nash warmed himself up in a patch of sun while Sura and Boerman analysed the survey images on the laptop. Clearly, they were just as confused, and after a short, muted argument, Boerman swore loudly and walked a short distance away to begin throwing stones down the dry river bed.

‘Mr Nash, would you come over here?’ Sura indicated that he sit on a plastic crate and smoothed her shapely thighs. ‘We have a conundrum. There is an Aichi Seiran torpedo bomber down there, which proves my submarine was once inside this cave. My question is – where on earth has it got to?’

Nash blinked at her expectant girlish stare. Did this entitled, self-obsessed maniac expect him to just pull a treasure map out of his arse?

‘Sura, I’m a cave diver, not a psychic. All I can tell you is water runs downhill, so my guess is somewhere downstream.’

Her expression darkened. ‘While you were down there today, I walked for nearly two kilometres along the dry river bed until I reached the resurgence of the river – a crystal-clear pool in which I saw brightly coloured fish swimming along the bottom. And I saw not a single shred of wreckage. Yet we have it on excellent authority the submarine was here.’

Nash stared at her curiously. ‘How can you be so certain?’

‘Because the mooring arrangement fits exactly, as does the seaplane left sitting on the catapult. You see, half the bullion was removed a few years after the war. But when they returned for the rest in the 1950s, the submarine was gone.’

Nash was growing interested despite himself.

‘And you’re certain it’s the Hoosenbeck Cavern? You’ve seen for yourself the lay of the land – there could easily be other caves.’

‘I can assure you we would not have come all this way otherwise.’

‘Maybe you should reconfirm with your source?’

‘No point.’ Her eyes were flat. ‘We don’t make those kinds of mistakes.’