This time, he picked up a basketball-sized rock before stepping off the shelf. The gravity assist preserved vital oxygen, and on the way down he pinched his nose to equalise the pressure in his ears. The light was a dull green when he landed on the bottom. He figured his depth to be around twenty metres. This was one hell of a passage.

Nash swam up at a forty-five-degree angle, looking to connect with the sloping wall of the sinkhole and stay orientated before the darkness closed in.

Doubt began interfering with his internal compass, and he stopped swimming and checked his illuminated watch. Just a few seconds to go before the alarm sounded. Gratefully, he turned back. By the time he cleared the underhang, his lungs were starting to burn. He forced himself to not race for the air, and broke the surface close to the two-minute mark.

The light was diminishing quickly, and he was cold. But he knew he hadn’t explored as far as his lungs could take him. This time, he breathed up for a full ten minutes, and reset his turnaround for one and a half minutes.

Diving straight and true, he took a direct line to the top of the exit passage and swam inside. The greenish glow behind him steadily gave way to grey, and then finally, pitch black. Now, he had to trust his internal compass. He swam on and on, gaining confidence until suddenly his fingertips encountered rock. Fear drilled through him. He wanted to turn back, but he needed to establish whether the rock was sloping down, or he was drifting up.

Turning turtle, he pulled himself along arm over arm. The roof was sloping up! Then the alarm went off.

‘Plan the dive, dive the plan’, the saying went. Nash should have immediately returned to the pool. But the rock was climbing so steeply, and he sensed he was almost at the surface.

Another five seconds passed . . . eight, nine, ten . . . The realisation that he’d probably passed the point of no return fought with the desire to turn back. But he had to be close . . . surely . . . He’d been ascending for what seemed like ages.

Have I miscalculated?

His head banged painfully into rock. Nash felt around desperately, finding an unbroken wall beneath his scrabbling hands. It was a dead end!

His final alarm confirmed it. Three minutes had elapsed. There was no point going back. Nash had less than thirty seconds to live.

With nothing to lose, he groped sideways for several metres. With every handhold, flashes of his life popped up unbidden: his mother’s hug when he graduated; his father’s quiet hand on his shoulder at Natalie’s funeral; Jacquie chasing him around the yard, aged four, with a water pistol. A great sadness filled him as he realised they would never know what had happened to him. And then his hand met nothing . . .