‘If you don’t mind.’ Azelio nodded at the broom. ‘Have you found the trick to it?’

‘Not really,’ she admitted. ‘Maybe what we need is some kind of covered system of barriers. If we can place it on the floor and then reconfigure it without opening the cover, we ought to able to manipulate the dust inside without any more arriving.’

‘That sounds . . . elaborate.’

Agata put on her corset and tool belt and followed Azelio to the airlock, then waited for him to cycle through. The view through the window showed that the weather was calm, but the Surveyor had become so filthy that Tarquinia now insisted on the protocol, regardless. Agata was beginning to suspect that the only remedy for the dust invasion would be to ascend into the void and flush every room out with clean air – and even that depended on their arrow prevailing and the void not being ready with a conspiracy of pollutants poised to rush in the moment they opened the airlock, in a perfect reversal of the intended purge.

Outside, she caught up with Azelio at the start of the trail. It was Ramiro who’d noticed the regularly spaced indentations in the ground after the last high winds, and decided to fill them with rocks marking the way to each of the four test plots. Agata hadn’t questioned him too closely on the matter, but she suspected that he’d already been contemplating doing something similar. The idea hadn’t come from nowhere, inspired by nothing but the evidence of its own implementation.

‘How are the calculations going?’ Azelio asked her, as they started along the trail.

‘Slowly.’

‘Just as well. If you finish them, what will you do on the journey back?’

‘There’s no risk of that.’ Agata had set aside her efforts to understand the curved vacuum and instead had spent the last two stints attempting to analyse their current situation, using a crude model of a field in which two opposing thermodynamic arrows met. But in the versions that were simple enough to handle, both arrows rapidly decayed away, leading almost immediately to a time-blind equilibrium state. The reality, in which countless slender fingers of opposing time interpenetrated, seemed to depend on details too subtle for her to approximate in any meaningful way.

‘It’s Luisa’s fifth birthday today,’ Azelio announced cheerfully. ‘I’ll show you her drawing for it when we get back.’

‘Happy birthday, Luisa!’ Agata played her coherer’s beam over the grey stones to her left. ‘You never peek, do you? You never riffle through the pile to see what’s coming up?’

Azelio buzzed. ‘Of course not! That would defeat the whole point.’

‘I know. But that wouldn’t be enough to stop me.’