Pao led the crew into the hollow mountain passage with the hand winch and timber braces. The route narrowed as it passed behind the furnace, leaving a stone slab wedged above the drainage door. If it shifted, it could crush the crew or block the exit.
Yan remained at the main wheel. Through the speaking tube, he asked Bai to take the spare chain and follow Pao’s measurements. “I can’t leave the pressure gate.”
“I’ll bring the chain to the marker,” Bai answered. “Pao knows the door.”
The passage floor was damp, but Bai found a dry bolt hole in the wall. He ran the chain through it and tested the anchor with a weighted crate before anyone stepped under the slab. The crate swung, then came to rest. The bolt held. Bai marked its position with chalk and asked Pao to keep the crew behind the line until the winch took the load. The crate swung, then came to rest. The bolt held.
Pao listened at the stone. “There’s a chamber behind it.” He tapped the wall with a small hammer. The sound came back hollow from one section and dull from another. Pao asked one of the engineers to mark the difference on the sketch. They followed the hollow tone for several paces, then found a second solid wall. If the door sat there, it was farther in than the map showed. Pao tapped again at shoulder height and listened for the return. “This side is filled in…
