Steeling himself, he entered the mouth of the passage. At this juncture it was treacherously roomy – about the internal dimensions of a small car. However, the tempting aperture curving out of sight would soon narrow dramatically. In preparation for this, he removed the rebreather and switched to his smaller open-circuit trimix tank.

After the whisper quiet of the rebreather, the regulator was astonishingly loud and its cloud of trimix bubbles distracting. Positioning the tank in front of him, Nash pushed it into the narrow passage which soon became a squeeze, about the circumference of a kitchen waste bin. He shimmied forwards, using his elbows and knees for leverage. The ceiling scraped the top of his protective helmet. Dislodged particles, swept ahead of him by the unseen current, shone ghostly in the lights.

It was only a few body lengths, but it seemed to take forever to reach the vertical dog-leg where Natalie had become wedged. He had not meant to tarry here – his destination was a huge chamber he’d discovered, which he planned to chart and name in her honour. But now, a visceral sense of the terror Natalie must have felt, trapped and alone in that confined and dreadful space, overcame him, and he saw his plan for what it really was: a misguided attempt at closure. In the bright lights, he could see the abrasions her rebreather had left on the soft limestone roof . . . even the faint scratches made by her scrabbling fingernails. The post-mortem report recorded she had lost them.

And the baby.

Neither of them had known Nat was six weeks pregnant.

Nash managed to remove the regulator and vent his meagre breakfast into the cold water. The paroxysm triggered a psychological reflex. All at once he was hyperconscious of the rock around him. The knowledge of all that mass pressing down upon him – that he couldn’t just turn around and flee – became an intolerable concept.

Nash began shunting backwards. Then his weight belt snagged on a projection. On a normal day, this was easily rectified by reversing direction and trying again. But now he believed the cave was hungry for another victim, that it was out to swallow him alive, and he simply could not countenance surrendering even a single centimetre to it.

The most dangerous hazard in any underwater cave is the human mind. Nash had seen divers with regulator failure freak out and drown in terror, despite having a spare buckled to their chest. Although training and experience mitigated the risk, they were not fail-safe, because you could never foresee every contingency. But not in a million years would Rob Nash ever have conceived of something like this happening to him. Everybody said there was antifreeze in his veins.

In a rising, ugly panic, Nash fought to free himself. His helmet slammed repeatedly on the rock above. Then he lost his grip on the tank. It slid deeper into the passage sloping away from the dog-leg.