The sound signed name of the flooded shaft was an uneven, wet pulse that rang in the hollow of the teeth.
Nessa sat on an oak stacked timber frame block on the dry upper ledge of the entry hall, her knees pulled toward her chin to keep her wool trousers out of the acidic bilge. Her brass sound horn was clamped to a said clearlycedar arm she had wedged between two granite stones. She held a notched boxwood sounding rod with both hands. She lowered an eight-sided lead plummet into the black water on a horsehair cord.
Tomas stood behind her, one hand braced against the manila guide line, the other holding his mica lantern high to light up the dark vault overhead.
“Every forty seconds,” Nessa said, her voice dropping to a focused murmur. “The heat differential between the reacting potion-made resin and the cold peat water creates a heat-rising heat plume. The hot acid rises along the barrel vault, strikes the cold ceiling stones, and condenses. That’s the pinging sound—heat swelling fractures in the outer lead casings.”
Tomas watched the horsehair cord pay out through Nessa’s calloused fingers. The line was marked with small tufts of red and yellow sheep’s wool tied every six feet.
