He shrugged. 'More than a hundred spans, but less than two hundred.'

'That's another hour, probably two. Can we make it before we breathe all our air?'

'I don't know.'

'I'll have to go faster.'

'Go too fast and it may tear the construct apart.'

'Too slowly and it won't matter' she retorted.

The minutes ticked by. Occasionally they came up against an object that scraped along the skin of the construct. It was hot inside now.

'How hot is the tar in the Great Seep, Merryl?'

'I wouldn't know. It's warm on top, so it must be warmer inside.'

'Hot enough to cook us?'

'I couldn't say.'

'Do you think we're at the end of the tar tunnel yet?' Tiaan asked.

'Once the node failed, the walk of the tunnel would soon have gone liquid. We'd be in the swirl of the Great Seep right now.'

'We're too slow,' she fretted. And we're not going up. I've got to do something.'

She knew what to do but was reluctant to do it, since that would give away the secret of making thapters-constructs that could fly. But if they were going to die anyway…

'Could you have the prisoners blindfolded, please, Merry!7 And ask the slaves to turn their backs. I've got to do something to the construct and I don't want anyone to see.'

He went down. Tiaan unpacked the set of pink diamonds -powerful hedrons – and the strands of black whiskers, fifty-four of each, weighing them in her hands. So much from so little.

'It's done,' called Merryl.

She lowered herself down the ladder by her hands and Merryl caught her at the bottom. Tiaan exercised her legs at every opportunity but it was going to take weeks before she could walk properly.

Opening a hatch in the floor at the front, she identified a black box among the tangle of parts inside, and prised the lid off. Inserting the diamond hedrons into their sockets, she fed the black threads up to the back of the amplimet cavity, checking everything carefully as she worked. There would be no time to do it again.