His wrist-mounted dive computer told him he was right on the edge of the threshold. By rights, he should turn around, now. But how long would it take to slip through and get a photograph? An advantage to this was that he could reverse direction to return.

Nash moulded himself to the contours of the rock, shimmying forwards on elbows and knees, pushing the tank ahead of him. Negotiating a super-narrow squeeze like this was something like birth. He had just pushed the tank through when he felt the single tug on the line. It meant stop, and Nash frowned. It was unusual for cave divers to pull on a line, because it could easily get stuck or break.

Confined in the squeeze, Nash gave her an awkward two-tug response: code for ‘I’m OK’. Then, gripping the edge of the slot with his free hand, he pulled himself forwards and popped out into open water.

The chamber was a vast half-hemisphere, so huge that in the periphery of his dive lights the deep blue water became impenetrable black. Nash was ecstatic. It was the elusive connection to Cocklebiddy!

He fired off a few quick shots with the GoPro and was turning to go when, suddenly, there were more tugs on the line. These were jerky, disconnected. Was she signalling ‘Come up’ or ‘Emergency’? Nash gave four quick tugs to signal he was returning. The responding tugs had no discernible pattern. And they seemed weaker. A chill came over him.

Nash found re-entering the slot much more difficult than he’d anticipated. After two abortive attempts, he literally drove himself inside with powerful beats of his fins. Powering up the slope towards the dog-leg, he was stunned by a shimmering ball of silver. It was a massive pocket of gas trapped at the apex of the sloping roof. At first, he couldn’t understand it. His open-circuit bubbles of trimix would have contributed nothing like this amount, and Natalie was on a rebreather, with no bubbles shed at all . . .

Her helmet lights were ominously still. Her floating arms were lifeless. When he got to her, she was unresponsive, mouth slack on her regulator mouthpiece. With agonising clarity, Nash knew instantly what had happened. Of much smaller build, Natalie had been able to enter the squeeze with her rebreather and it had caught in the dog-leg. While she struggled to get free, her breathing loop hoses had torn, venting the contents of her oxygen and diluent tanks in a catastrophic gush.

Reaching around, he retrieved her secondary mouthpiece and forced it into her mouth. Depressing the button, he expected a blast of trimix from the bale-out tank on her thigh, but there was nothing. She’d taken off the backup to fit inside the squeeze.

Quickly, he pushed his mouthpiece between her slack lips. Her cheeks expanded with gas, but her eyes stared sightlessly.