She made her way up-axis as swiftly as she could, advancing through the blackness, searching the walls for patches of red. Whoever had worked this section before her had been diligent; she saw only the tiny specks of new growth, easily disposed of with a quick flash from her coherer. As far as she knew no one else would be coming here now, but she’d resolved to do a passable job every shift in case there was an unannounced inspection. If her rushed work wasn’t quite as thorough as that of her younger colleagues she could always blame her failing eyesight, but there could be no excuse for great glowing colonies of moss.

She reached the end of her allotted segment of the tunnel with almost a bell to spare, then she turned and raced back towards the start. The hard part was doing it quietly, keeping her feet low and lengthening her strides instead of breaking into a run. People were used to hearing workers in the tunnels, but the sound of outright sprinting might attract attention.

Light from the open hatch marked her entry point, but when she arrived at the ladder she slid the hatch closed above her and waited for her eyes to readapt to the dark. A couple of strides down-axis from the hatch, a hardstone grille covered the width of the tunnel. Peering between the bars she saw nothing: no flashes from the coherer of another tunnel worker burning off moss. Agata hadn’t quizzed Celia about anyone else’s shifts, but she’d chosen the latest of the time slots on offer. It was possible that right now there was no one at all between her and the cooling chamber.

She lay on the floor of the tunnel and rearranged herself so that she could reach down her throat and retrieve the small bundle of tools she’d swallowed. The rags they were wrapped in were covered with clumps of food and digestive resin; she shuddered but managed to avoid emitting a hum of revulsion as she flicked her hand clean.

By touch, Agata confirmed what she’d guessed by sight when the grille had been illuminated: the bars really were embedded in the wall, continuing right into the surrounding stone. The masons must have drilled one hole straight, but created a whole triangular cavity at the opposite point, allowing a rod that was too long to fit directly across the tunnel to be inserted at an angle and then made true. They would have packed the cavity with sand and adhesive, and over time it would have set into something almost as strong as the surrounding calmstone. But though the bars themselves were close to unbreakable, calmstone was just calmstone. Agata put the point of her hand drill against the wall beside the longest of the bars and set to work enlarging the hole that contained it.