Minus Four sported a massive round hatch, a meter in diameter, let into the wall of that cylinder. Minus Five had nothing but routed systems of the smaller and more intricate type: sensors, probably networked to computers elsewhere. At Minus Six, the ladder stopped and they bottomed out on a natural limestone floor. "This is the base of the excavation," T.R. said. "This is all the farther we dug. And this here"—he rapped a knuckle against the cylinder wall—"is the base of the cylinder, where the methane burns."