As they descended the ladder in the red-tinged gloom, Agata could hear the muffled hiss of gas in the tunnel beside them. No one came here on regular cleaning shifts; the warm air was inimical to moss.

When they arrived at the bottom of the shaft the darkness was impenetrable. Serena said quietly, ‘It’s us,’ and someone switched on a coherer. Agata squinted into the glare and counted two dozen and nine figures squeezed around them, already wearing their corsets, cooling bags and jetpacks. Many of them had never used the jetpacks; they should all have had one-on-one briefings earlier from their more experienced friends, but it was Agata’s job now to go through the safety checks and remind them of everything they’d forgotten.

‘If you get into trouble,’ she began, climbing two steps back up the ladder to make herself visible to everyone, ‘just draw a stop line: a straight horizontal line across your chest.’ She demonstrated. ‘The rock will still be moving below you, but don’t let that confuse you: the pack will bring you to a halt relative to the mountain’s axis, so you won’t go flying off into the void.’

There was no time for more than the basics, but if they could retain it, it ought to keep them alive. Agata put on her own equipment.

‘Does everyone understand what we need to do with the occulters?’ The protocol she’d written had been copied discreetly from skin to skin, and some of the volunteers would not have received it until they’d reached this assembly point. In a perfect world they would have rehearsed the manoeuvre daily for a stint or two, but at least the jetpacks would handle most of the navigation.

‘Can the machines drill into our bodies?’ a young man asked anxiously.

‘Not intentionally,’ Agata assured him. ‘They’re not that sophisticated; they have no defensive strategies as such. The only danger is if they’re so confused that they mistake you for rock, but if you get out of their way they won’t pursue you.’

Serena passed Agata a helmet. They were aiming not to use the links; this would probably be their last chance to talk until they were back in the mountain again.

‘Happy Travellers’ Day,’ Serena said.

‘Happy Travellers’ Day,’ Agata replied. She put on her helmet and turned towards the maintenance hatch.