He eyed her mockingly. ‘You’re here to destroy me. I always knew you would finish up like this. You actually liked running the mazes the Qax built for us - the tests, the meaningless career paths, the competitions between the cadres. Even the Extirpation is just another pleasing intellectual puzzle to you, isn’t it, in a lifetime of puzzles? Oh, the Qax are smart rulers; they are exploiting your talents very effectively. But you don’t have any idea what your work means, do you? . . . Come with me.’ He grabbed her hand, and pulled her towards the curved electric blue wall of the facility.

She shivered at the remembered warmth of his touch. But he was no longer her cadre brother; he had become a ragamuffin, one of the dwindling tribes of humans who refused to remain in the Qax Conurbations, and his face was a mask of set planes and pursed lips, and his determined anger was intimidating.

To get to the Qax facility they had to walk through the shanty community. It was a pit of rough, improvised dwellings, some little more than heaps of sheeting and rubble. But it was a functioning town, she realised slowly, with a food dispensing plant and a clinic and a water supply, even what looked like a rudimentary sewage system. She saw a small, dishevelled chapel, devoted to some no-doubt illegal religion, whose gods would one day free humanity from the rule of the Qax. All of this was laid over a mighty grid of rubble. There were still fragments of the old buildings, bits of wall and pipe poking like bones from the general wash of debris, some scarred by fire. Where vegetation had broken through the concrete, the remnant walls had become low hummocks coated with thick green blankets.

There was a stink of smoke and sour humanity, and the air was full of dust which clung to her skin and clothes. It was hard to believe that any cadre sibling of hers would choose to live here. Yet here he was.

Symat was talking rapidly about superheavy elements. ‘It used to be thought that marsdenium and its more exotic sisters could only exist as technological artefacts, manufactured in giant facilities like this Qax factory. But now we know that such elements can be born out of the great pressures of a supernova, the explosive death of a giant star.’

She tried to focus. ‘An exploding star? Then why are you looking for heavy elements here on Earth?’

He smiled. ‘Because the Earth coalesced from a cloud of primordial gas and dust, a cloud whose collapse was triggered by the shock wave from a nearby supernova. You see? The primordial supernova laced the young Earth with superheavy matter. So the heavy elements have deep significance, for Earth and all that live on it or in it.’