The sump gave out an hour before dawn, and it gave out badly.
Thomas heard the change before he understood it — a shift in the pump-rod's rhythm from its usual heavy gasp to something faster and thinner, almost frantic, and by the time he'd gotten down the stairs with a lantern the water in the undercroft was already past his ankles and rising.
"Packing's blown on the lower joint!" Wat's voice came from somewhere in the dark ahead, half-swallowed by the roar of water finding a new path through stone that had held it for a year. "Get the boy up top and close the sluice-gate before the whole race empties into this hole!"
Thomas didn't argue. He shouted the order up the stairwell and waded forward into the freezing black water toward Wat's lantern, his boots filling instantly, the cold hitting his shins like a physical blow that made his whole leg go momentarily numb and useless.
"How bad?"
"Bad enough that if you'd come down five minutes later I'd be swimming for the terminal board instead of standing on it." Wat had both hands braced against a section of failed oak packing, holding back a jet of water with nothing but his own weight and the stubborn refusal to let go. "I need the spare hemp caulking and I need it now, and I need three more hands, because I've only got two and they're both busy."
Thomas didn't have hemp caulking on him. What he had was a coil of resin-soaked linen wrap meant for the copper lines, and no time to find anything better. He shoved it into the gap beside Wat's hands, felt the pressure of the water fighting them both, and held on.
"That's copper insulation, not packing," Wat said, through his teeth.
"It's what we've got."
Above them, the sluice-gate groaned and dropped, and the roar of incoming water began, gradually, to ease — not stop, but slow to something they could fight instead of something that was fighting them. They held the makeshift packing in place, soaked to the chest, saying nothing, until Wat's apprentices arrived with real caulking and pried them both loose to do the job properly.
Afterward, sitting on the stairs with a blanket someone had produced from somewhere, Wat looked at the ruined coil of copper-wrap Thomas had sacrificed to the leak and shook his head slowly, more impressed than he probably wanted to admit.
"That was a fool's fix," he said.
"It held."
"It held," Wat agreed, "which is the only reason I'm not telling you what a fool's fix it was in more detail." He wrung water out of his beard with one hand. "You've got good instincts for a man who spends half his life talking to a stone. Next time, though — hemp's in the second chest from the door, marked with a red cord. Learn where it lives before you need it in the dark."
Thomas laughed, tired and cold and entirely too aware of how close the night had come to going differently. "I'll remember."
"See that you do. I'd rather not explain to your wife why I let you drown saving a spool of wire."