‘We’re travelling up, I take it,’ Peter said.

‘Wherever we can. It may not always be possible. But this is a good start, and the current of air is from above. Is everyone sorted out? We want to try to keep on the move as long as we are using light.’

Heather said: ‘We couldn’t have a little snack first? I’m truly truly ravenous.’

‘We will eat at the beginning and end of the day,’ Henry told her.

‘Can’t we call it the end of the day? It’s black enough, surely.’

Henry looked at his watch. ‘It’s four o’clock. We shall call it a day at nine, providing we can find a reasonable place to sleep.’

‘Sleep!’ Heather echoed. ‘Down here? On bare rock?’

‘I think you will be tired enough by that time to sleep anywhere. Especially after feasting on a couple of squares of chocolate. I’m dividing all of the rations to last three days.’

‘Three days!’

Henry looked at her. ‘After that, we’ll have to tighten our belts.’

‘But we aren’t going to be three days down here,’ Heather protested. ‘The idea’s silly.’

‘I hope you’re right. But I’m not taking any chances. If we make it sooner, we’ll have an orgy.’

‘I’ve always liked orgies,’ Heather said.