The shoes kept walking. Just before they disappeared from view, I clicked the shutter. They were still only tiny on the display screen, but I knew Amelia would recognise Caleb’s stupid boots. As soon as I tilted the screen her way, she did.

‘Why didn’t I think of him earlier?’ she said.

‘It’s no use,’ I muttered. ‘He can’t hear us.’

She looked at me with a spark of the usual are-you-actually-an-idiot glinting in her eye. I was genuinely relieved to see it, as it generally means she’s thought of some clever idea. This solution was actually simple enough to make me feel stupid. Amelia pulled her phone from its case and called Caleb’s number.

It’s idiotic, but the fact that he was already stored in her contacts made me shiver, despite the ridiculous heat.

‘Hey, Caleb,’ she said beside me. If there had been panic in her voice a moment ago, she now sounded ridiculously matter-of-fact. ‘Are you wearing your very green boots?’

I was still pressed up against the drill hole with my camera, so couldn’t hear his reply.

‘Cool. And are you at work,’ she went on, ‘doing whatever they’ve got you doing at your father’s mine?’

The truck hadn’t moved, and neither had anything beneath or beyond it.

‘Cool,’ she said again. ‘Because we’re here too and we could sort of use some help.’

Cool? Sort of? She wasn’t just trying to be matter-of-fact, I realised. She was trying to sound off-hand! ‘If you give that to me, I’ll talk him to us,’ I said as levelly as I could, reaching for the phone. I pretty much had to prise the thing from her fingers.

‘Yeah, hi, Caleb, it’s me. We’re locked in a container. It’s the third one of four in a row along the inside of the northern perimeter fence, on the left as you come through the main entrance to this mine-complex place. I just saw you, through a tiny hole in the metal wall. You were walking away from us, behind that truck with all the concrete tubes on it, heading east. If you could –’

‘You’re locked where?’ he cut in.

As patiently as I could, I said, ‘Just retrace your steps, quickly, please. We’re melting in here, seriously.’

I was still looking through the hole. When his shoes appeared again, tiny ticks of lime-green hope, I let out a breath. ‘Stop,’ I said. The shoes stopped. ‘Turn left, towards the main entrance.’ Astonishingly the shoes turned right, heading for the hut. ‘Left!’ I said. ‘Round the back of the truck.’ The whole of him, phone held out like a little tray in front of his chin, the way idiots use them in the street sometimes, came into view. ‘That’s it. Now look to the north. You’ll see the containers.’