Alicia shut her eyes. The water grabbed for her ankles and retreated, trying to take her with it. She buried her fingers in the folds of Finn’s jacket. “Did you live here?”

“No, we lived somewhere else, but we used to come here on holiday. And, as I said, at the time I was really, really keen on keys. I thought they were just the best, coolest things in creation. So before we went away, I went out to the back garden and took the spare key out of the flowerpot. My mum and dad weren’t keen on me having keys because they thought I might lose them. So I hadn’t told them I’d got it. And here we were in the cave, and our back door key was just sitting there, in the pocket of my jeans. Steady, don’t slip. Hold onto me.”

Alicia clutched at him, trying not to sob in terror. “What are you talking about, what’s this got to do with anything?”

“My mum and dad went back to the beach, but they said I could stay for a bit if Ava stayed with me. Ava wanted to go back, but she stayed with me because she’s nice. So I was climbing around in the very back of the cave, and – oh my God, look at this.”

He held out his hand and shone the torch-beam onto a silvery, rust-flecked mortice key.

“What bloody good is that, what a bloody stupid thing to… ”

“No, you don’t understand,” said Finn. “I left it to remind myself. Now tell me the truth and I swear I’ll stay right with you whatever you say, because either answer’s fine. Do you want to call it a day and give in, or do you want to try and get out of this?”

“What?”

“We’ve got about one minute more before the tide makes the decision for us.”

“I – oh, look, I want to live, all right? I want to bloody live, I thought I didn’t but I do, I thought if I died it would fix everything but… ”

“Then reach up here where I left the key.”

She stretched her hand up as high as she could and found a deep empty hollow. Finn shone the torch into the darkness. Above them, hidden unless you shone the light at exactly the right angle, was a square tunnel sloping upwards and backwards into the cliff.

“It’s the bottom of an addit,” Finn said. “You know the tunnels that drain groundwater from mines? I don’t know how far it goes, because I only got a couple of feet in and then Ava made me come down. She said people poking about in old mine workings was how horror movies started. But I think this might be the night to take our chances with the Morlocks.”

“I can’t get up there, it’s too high, it’s too steep, it’s too wet.”

“And… there go my boots. We’d better try, or the next few waves’ll have us. Come here, I’ll give you a boost.”