Dolo said, ‘It’s a common enough reaction. The troopers have little control of their lives, even of their deaths. So they seek to control what they can - like the ground they dig, the walls that shelter them - and they come to believe that such actions in turn might placate greater forces. All utterly non-Doctrinal, of course.’

Luca snorted. ‘It is a sign of weakness.’

Teel said without emotion, ‘Imagine this Rock cracking like an egg. Sometimes that happens, in combat. Imagine humans expelled, sent wriggling defenceless into space. Imagine huddling in the dark, waiting for that to happen at any moment. Now tell me how weak we are.’

‘I’m sorry,’ Luca said, flustered.

Dolo was irritated. ‘You’re sorry, you’re sorry. Child, open your eyes and close your mouth. That way we’ll all get along a lot better.’

They walked on.

The horizon was close and new land ahead hove constantly into view, revealing more pits, more toiling soldiers. Luca had the disconcerting sensation that he was indeed walking around the equator of a giant hall of rock, and his vertigo threatened to return.

It was because he was so busy trying to master his queasiness that he didn’t notice the arch until they had almost walked under it. It was a neat parabola, perhaps twenty metres tall. A single trooper was standing beneath it, hands behind her back, stiffening to attention as Teel approached.

‘Ah,’ said Dolo, breathing a little heavily with the exertion of the suited walk. ‘So this is what we have come so far to see.’

Luca stood under the arch. Its fine span narrowed above him, making a black stripe across the complex sky. The arch was so smoothly executed that he thought at first it must have been erected by machine, perhaps from blown rock. But when he bent closer he saw that the arch was constructed from small blocks, each no larger than his fist, stone that had been cut and polished. On each block writing was etched: names, he saw, two or three on each stone.

Teel stood at one side of the arch, picked up a pebble of conglomerate, and with care lobbed it upwards. It followed a smooth airless arc that almost matched the arch’s span. ‘Geometrically the arch is almost perfect,’ she said.

Dolo bent to inspect the masonry. ‘Remarkable,’ he murmured. ‘There is no mortar here, no pinning.’

‘It was built by hand,’ Teel said. ‘The troopers started with the keystone and built it up side by side, lifting what was already completed over the new sections. Easy in microgravity.’

‘And the stone?’

‘Taken from deep within the asteroid - kilometres deep. The material further up has been gardened by impacts, shattered and conglomerated. They had to dig special mines to get to it.’

‘And all done covertly, all kept from the eyes of their commanders.’

‘Yes.’

Dolo turned to Luca. ‘What do you make of it, boy?’