‘I like it,’ Agata replied. ‘It’s meditative.’

‘That’ll wear off,’ Celia promised.

Agata turned into a dusty corridor and followed it for a few stretches. The entrance to her section of the cooling system was easy to find: moss had eaten a great concave chunk out of the wall beside it, and the usual red glow was criss-crossed with threads of yellow. She slid the key into the access panel and strained to pull it open.

A short ladder led down into the tunnel. The breeze pulsing through the darkness chilled Agata’s skin, but after the first shock it wasn’t unpleasant. The ceiling of the tunnel was too low for her at her normal height, but she only had to resorb half a span from the top of her legs and she fitted comfortably. She started walking, one hand on the wall to guide her. The splotch of red light spilling in from outside shrank in her rear vision, and when it was gone she was in utter blackness.

A saunter or two down-axis, in a set of reaction chambers carved into the lode of sunstone that had once been destined to be burnt as rocket fuel, a decomposing agent was turning that fuel directly into gas – without the usual accompaniment of light and heat. The gas built up a considerable pressure, then as it forced its way out against the resistance of a spring-loaded piston, it grew colder. This was much more efficient than the old system that had used the exhaust from burning fuel as its starting point, but the moss that coated the mountain’s walls grew so vigorously beneath the new kind of breeze that it threatened to clog all the cooling tunnels.

As Agata’s eyes adjusted to the darkness, she began to see the small red patches of new growth around her. A shift ago, she’d left this whole section as bare rock, but it didn’t take long for fresh spores to blow in and find purchase. She took the coherer from her tool belt and aimed it at the nearest colony, closing her eyes against the flash so they wouldn’t lose their sensitivity.

Three stints into the strike, she still found the job absurdly satisfying. The work was vital, and she could see the results of her efforts immediately. That the moss returned so quickly didn’t bother her at all, so long as she could keep it under control. Better to walk the length of the tunnels regularly, searching for these early infestations, than wait for the walls to become so encrusted that they’d need to be scraped clean with a hardstone blade.