Fry drew back from the opening, covering her nose and mouth against the stench.

‘You can’t do this,’ she said.

But Cooper ignored her, concentrating on climbing over the slimy edge of the hatch and feeling for the top rungs of the ladder. As he clambered carefully down, the counterweight for the hatch bumped against his back, tap-tapping like a heavy hand on his spine, on his shoulders, and touching the back of his head as his feet touched the water. Then he looked up again at the light, saw Fry silhouetted against the sky, her coat and hair filmed with rain.

‘I know the layout. I’ll be OK.’

‘How can you know it?’

‘They’re all the same. A standard design.’

‘You don’t know what else might be down there.’

‘I’ll be careful,’ said Cooper.

‘Famous last bloody words. You’re mad.’

Cooper summoned his recollection of the Edendale post. Beneath him was the bottom of the shaft, behind him an overhang and a wooden door – the chemical toilet and generator room. To the left would be the other doorway, into the monitoring room. He could see the top of the frame, was relieved to see that the door stood open.

If he had been Michael Clay, trapped down here with the water rising, where would he have made his way to? Where would have been the best place to eke out the last bit of remaining air? The shaft itself, surely? There was a good six feet of space above the water line.

But he touched the walls and felt how wet they were. Slippery with a foul-smelling sheen of mud and mould. So the level of the water was actually falling. At its peak last night, or in the early hours of the morning, the shaft must have been flooded right up to the top, only the locked hatch preventing water from seeping out on to the surface.

So if Michael Clay had known the layout of an ROC bunker, what else would he have done? He would have gone for a ventilation outlet. Of course. Cooper pictured a rusty louvred steel opening in the far wall of the monitoring room. And somewhere in the ceiling was the lower end of the blast pipe, wide enough to detect the pressure from a fireburst explosion, so it must allow the passage a bit of air, too.

Cooper sucked in a long breath and ducked his head under water, pulling himself towards the open doorway. Moving into pitch darkness, he was blinded by the sudden contrast with the light in the shaft and its splintered reflections on the surface of the water. He was so disorientated that he had to break the surface and take a new breath, panicking for a moment that he wasn’t going to be able to do it, at the thought that he would have to admit defeat and go back up to the surface, just sit and wait for the experts with their wet suits and oxygen tanks, which could take forever.