They passed from the storeroom into the chamber which had formerly contained the hoist. Here they noticed a tall heap of earth.
"They dug that out when continuing the underground passage to the bridge," said Pariset.
"Here's the trap-door," returned Kenneth. "Look! There's a wire running through it, connecting with the room behind."
"It's all very thorough, confound them!" said Pariset. "I hope the trap-door won't creak."
They lifted it gently, and found that it moved on a central axis, well oiled. Peering into the dark depths, Kenneth discovered a wooden ladder. They crept down this, into a large underground chamber flagged with stone, and ventilated by narrow gratings in the brick walls, above the level of the stream.
"We had better not both go on," said Pariset. "I'll go up and keep watch. You proceed, and cut the wires at the further end of the passage."
"Why not here?" said Kenneth. "It would save time."
"But if the word should come to fire the mine, and they find the apparatus doesn't work, they'd soon discover the cut here and repair it. Much better do the damage at the other end."
"Very well. You'll use your revolver if they come before I get back?"
"Yes. I'll take my chance. They probably won't guess that there's any one below, if I shut down the trap-door. You know what to do: cut the wire, or disconnect the terminals."