“That’s classified. So, the post-box. We never thought to question how there could be a post-box that didn’t work, or what happened to all the post that went into it. To be honest, we were both really pleased to be told about it. It was like getting this brilliant piece of insider knowledge. We used to feel quite sorry for all the people who didn’t know about the post-box not working.”

Alicia’s hand, reaching blindly for the next hold, encountered instead a terrifying void. The sound of her shriek was so appalling that she frightened herself. She thrashed round wildly in the tunnel, slipping a few inches, then wedging herself back in.

“I’m stuck, I’m stuck, help me, I’m stuck, oh fuck, this is it, I’m fucking stuck.”

It was Finn’s hand, holding firmly onto her ankle and giving it a comforting squeeze, that reminded her she was not alone. She forced herself to be quiet and breathe, trying to get control of the panicked gasps and whimpers.

“I put my hand up and there was just this space,” she stammered out.

“Really? Maybe it’s a chamber. Climb back up and see if it’s got a floor. Make sure it’s not a mineshaft first.”

“I can’t. I can’t. Let’s stay here.”

“I took some shaggy ink-cap mushrooms in to school once,” said Finn. “Not just out of badness, for the nature table. And the teacher put them on the display, and after four days they went all into ink and made a massive black stain on the tablecloth. So she made me look at it, and she said, ‘What have you got to say about this, then?’ And I said, ‘Well, that’s why they’re called shaggy ink-caps.’ Then she said I was being cheeky and sent me to the headmistress. I know that’s not much of a story, but I’m not sure we can both hold on to a damp slippery rockface for the next six hours, so why don’t you have a feel around and see if you’ve found somewhere we can wait out the tide?”

The strain in his voice frightened her more than anything else. She inched her way back up. Once again her hand found nothingness. She reached in with her arm and felt around. A flat damp floor. A room. A sanctuary. She shuffled forward onto her stomach and began to wriggle gladly forward into the void.

“I don’t want to sound like the voice of doom,” said Finn, “but do make damn sure you don’t tip yourself down headfirst down another addit, okay? Just find a wall if you can. And then keep dead still until I get up with the torch.”

She crept in on her belly, then banged her forehead painfully against a wall of rock. She pressed herself gladly against it, then stretched her hand out in panic. Was there a drop right next to her? What if she fell asleep and – a wavering finger of light crept into the cave, followed by Finn, grazed and grimy and wriggling like a caterpillar.