‘The overflow will hit us very soon,’ Nash told Mia as they were lowered into the vine-shrouded overhang. ‘Whether Sura flies or not, all we can do now is buy time –’ He broke off as frigid run-off from the sinkhole edge coursed over them and made her gasp. ‘Use this pony tank and get underwater. Stay shallow and use the submarine as cover. Use the sinkhole walls, too, find underhangs to stay out of sight.’

‘I can’t stop thinking about Frank.’ He could feel her trembling against him.

‘He’ll know what to do.’

Nash felt terrible leaving the old man, too. He figured Douglas would take as many as he could to give them a chance. But would it be anything like enough?

They passed through the log pile and between their dangling feet the flattened stern of the I-403 came into view, a rusty peninsula surrounded by jade-green water. A soldier was squatting by a square hole cut aft of the deck gun with the oxy. Dense mist rose up as the water met the warmer air.

‘See how turbid it is?’ Nash said, as they swayed back and forth. ‘They won’t be able to see you underwater.’

‘But how are we ever going to get out of here?’

Mia was staring, wide-eyed, at the vertical walls and the impossible spectacle of the inverted submarine. Her fear was mounting with good reason, but he needed her not to panic.

‘It’s going to fill fast, remember?’ He kissed her cheek quickly and tried not to think of water temperature and all the other variables ranged against them. ‘Just focus on staying alive, and we’ll float to the top.’

A young NCO with darting eyes helped free them from the tangles of the cargo net – no easy task at a twenty-five-degree angle. Rapping Nash’s equipment with the barrel of his pistol, he yelled, ‘No room! No room!’

Nash quickly hooked the rebreather over the breech of the deck gun and wedged his fins into a raised edge of broken plate. With a nod to Mia, he clipped the pony tank on, too.

‘You go, you go!’ The NCO waggled his pistol at the rough-cut hole. It looked mean, dark and dangerous. ‘You go, you go!’

The NCO’s voice rose in pitch. For a moment Nash seriously contemplated trying to seize the weapon, but then he saw the upturned face of another armed soldier waiting inside the submarine.

The hole was less than a metre square, and Nash had to squeeze his broad shoulders through before dropping into thigh-deep, freezing water. LED lights illuminated a steep and dangerously corroded passageway which ran on through another bulkhead. Then Mia’s shapely legs appeared. After helping her down, Nash took her by the hand, and they followed the soldier, sloshing through the water until they stepped through a bulkhead door into a vast engine bay stinking of grease which, thankfully, was still dry.