"If you have two parallel drifts, one above the other like storeys in a building, then the rock in between them—the floor or the ceiling, depending on which way you look at it—must be thick enough to support itself. In the structure we are walking through, the rock is just barely thick enough. But when the demolition charges have been set off, the rock will be shattered so that reconstructing these drifts will be a physical impossibility."