With a glug, the hole went under. Nash thought he heard a shout of terror as the first spout of water jetted into her compartment. The thought of Mia trapped in there by a couple of cheap fucking cable ties was driving him mad, and an involuntary sob racked him as he cut off the sleeve of his drysuit and wedged it into the hole to try and buy her a few more minutes.

He reassured himself the pony tank was still attached to his weight belt. Its miserable 540 litres of compressed air would have to save both their lives now.

With water now swirling about his ears, Nash pressed his pursed lips against the rough bulkhead to suck in every last precious lungful of air. Then, with a silent goodbye to Mia, he flipped over and pushed back down the flooded passageway. Hand over hand, he dragged himself down, kicking his feet in a steady swimmer’s beat.

Swimming clear of the submarine, he encountered a milky green fizz which blocked any clear indication of the surface. But it no longer mattered, for Rob Nash was not going there.

His plan was not really a plan – rather, speculative ambition. In fact, the only thing that exonerated it from the realm of the suicidal was the belief that survival was theoretically possible, as opposed to certain death at the hands of a professional killer above.

Unclipping the pony tank, Nash slid its regulator into his mouth. At his current depth of fifty-four metres, one cautious breath consumed ninety litres, or one-sixth of his air supply. Instantly, his lungs began to burn, and he had to clamp down on a coughing fit. While the compressed air he was breathing was proportionally identical to its surface composition, its partial pressure had increased, meaning the percentage of gas he was absorbing had increased to dangerous levels. Not only was the nitrogen hit on his nervous system equivalent to consuming seven full-strength beers; the concentrated oxygen searing his alveoli was a precursor to a convulsion. His precious air supply was now a toxic soup.

It was now that he caught his first blurry sight of the exit passage – a black maw, swallowing driftwood and other debris. He had a scant second to register that the ledge from which he’d free-dived had been swept clean of rocks before the incredible power of the Hoosenbeck swallowed him, too.

It was a giant water slide – a moment of negative g, followed by sickening acceleration. Nash clutched the pony tank to his chest and curled into the tiniest ball he was capable of forming. Such was the velocity of his descent, he thought he was about to be dashed into the floor of the passage. A second later, he was hurled upwards, in an out-of-control fun park ride, somersaulting end over end.