‘Grab something I can’t even see?’

‘You got any better ideas?’ Kenny kicks across the tank to the northern wall, then sidestrokes slowly from one corner to the other, measuring the length of the wall in handspans, counting each aloud.

‘What the fuck are you doing?’

‘Shut up – now I have to start again.’

Tom keeps his mouth shut, compliant for the first time that night, perhaps for the first time in all their years of friendship. Reaching the end of the wall – a hundred and seventy-two spans – Kenny counts back half that number and scratches an invisible groove into the soft rind of slime above the water level.

‘What if the ladder’s off-centre?’ Tom asks.

‘Just get over here.’

Kenny treads water below the imagined ladder, willing it to be there. He clasps ten fingers together to form a foothold, Tom plants his right foot into this makeshift stirrup and heaves himself up. The weight submerges Kenny completely; he surfaces to find Tom a few feet out, breathing hard.

‘Nothing there.’

‘Two steps this time – one onto my hands, then up onto my shoulders.’ The force of the recoil submerges him again, more deeply. After regaining his breath, they try again. ‘We’re doing this the wrong way around. I should be on top.’

He launches himself upwards from Tom’s clasped hands, his own hands finding nothing in the darkness but the dry, crusted wall. Tom’s panicky voice is echoing through the chamber as Kenny surfaces after the fifth or sixth such attempt. ‘I’ve got a cramp.’

Kenny is hit in the face by a flailing arm.

‘Kenny? Give us a hand. I need to hold onto something.’

‘Lie on your back. All you have to do is float.’

Tom rolls onto his back, but almost immediately rolls over again. ‘Jesus – now the other one is seizing up. All that fucking jumping.’

‘Float on your back. I’ll stretch your legs.’

But Tom is grasping at him again, frantic. ‘Just let me hold on for a minute.’

The weight of his bigger friend submerges Kenny; he kicks free underwater, on the edge of panic himself. ‘Where are you?’ Tom is shouting as he breaks surface. ‘Kenny? Where are you?’

‘Keeping my distance unless you do what I fucking say.’ This time the words sink in. Kenny supports Tom with one hand beneath the chin, keeping the mouth and nose just above the surface. He has to work harder himself, treading water at jogging pace, and knows he cannot continue indefinitely.

But Tom is in some pain. Breathing heavily, his words come in gasps. ‘Jesus, Kenny, what are we going to do?’

The question shocks Kenny; it seems too general, too open-ended. Far better to stick to particulars. ‘Panic makes cramp worse. It’ll ease if you stretch the legs.’

‘It’s not easing.’

‘It will. Try to ignore it.’ His mind gropes for solutions. He tries to remember the lifesaving lessons, three long years before. Half the battle is mental – is that what he had been taught? ‘You know,’ he says in Tom’s ear, ‘You still haven’t told me.’