It was a ledge! Dragging himself around, Nash got his feet planted. Lungs on fire, he pushed off with all his might. One final hurrah, straight up like an arrow.

A scant second later his head broke the water.

Air, sweet, blessed air!

Nash sucked it in, deep draughts echoing in a perfect void of blackness, and by the volume of his echoes, he knew it was a large chamber. He swam for several metres, until he encountered a sloping rock surface. Jittery with cold and adrenaline, he clambered out.

His next moves were critical. He was reasonably certain that he could follow the mental route back and find the green glow of the entrance pool. But the longer he stayed here in the pitch black without any visual reference points, the faster that fragile mental map would degrade.

Nash had one precious source of light – his watch – but the battery would not last long. He activated it just long enough to absorb the topography within the illuminated five-metre radius and was heartened to see the slope continued up. Fortunately, there were loose rocks and branches, detritus from the last flood event, and without ever taking more than three steps from his position, he gathered these by feel, building a succession of small cairns. In this way, he laid a return trail, taking perhaps half an hour to travel fifty metres.

His heart sank as the rock began sloping down again. Sure enough, he soon hit water. Bitter disappointment coursed through him. How long did this next sump continue for? It might have been a few metres, but in absolute darkness, without any point of reference, it might as well have been twenty kilometres.

What he should have done next was to immediately head back while there was enough daylight to guide him to the entrance pool. But Nash was suddenly overcome by his predicament, and slumped down in exhausted despair. What was out there anyway? The whole ramshackle edifice of his life was one he no longer recognised – one he didn’t know how to live. Perhaps being marooned on the shore of a netherworld beach was a fitting way for it to end?

‘Sonofabitch!’ he roared in the sightless grotto. ‘It wasn’t my fault, Nat! Do you hear me? It wasn’t my fault!’

A wave of self-pity and frustration engulfed Nash. Everyone had been so quick to blame him. But Natalie was an experienced cave diver, and she had made the error coming into the squeeze, not him. The whole dive had been dissected by the coroner, who had ruled it an unfortunate accident. And yet, why didn’t the formal exoneration give him any comfort? Why had saying sorry to Brendan and Jonathan felt so fucking hollow?

‘What do you want me to do, then, Nat?’ he said wearily. ‘Join you? Go on, then, give me a sign.’