He takes them to a place where an iron manhole is set into a hole in the rock wall, sealed in place with tar. "There are a dozen of these," he says. "Each one leads to the Lake Yamamoto shaft—so pressurized water will be behind it. The only thing holding them in place right now is tar—obviously not enough to hold back the pressure of the lake water. But when we have filled these rooms with sand, the sand will hold the manholes in place. But if a thief breaks in and removes the sand, the manhole explodes out of its seat and millions of gallons of water force their way into his excavation."