The reduction in the intensity of the Hard Rain would have been obvious to them on one level, since they could hear the impacts through the walls of the Hole. On another level it was easy to miss, since even dramatic changes spanned generations. But they had kept meticulous records of the frequency and intensity of strikes and so they recognized the downward trend in the late Fourth Millennium. When it was adjudged safe, they drilled an adit—a horizontal tunnel—out the side of the mountain until it broke out of a slope that they guessed was steep enough to have shed ejecta, preventing the buildup of rocky debris that now covered most of Earth’s surface to a considerable depth. That much had been true, but the debris at the base of the mountain had piled higher than they had expected—almost high enough to block the opening of their adit. Anyway, it had worked well enough that they had been able to push spoil out of it and thus begin expanding the Hole. The atmosphere was still far from breathable, so they’d been obliged to keep it sealed when they were not actively dumping stuff out of it, to prevent fumes from seeping in and poisoning the atmospheric system that they had looked after so meticulously for nearly four thousand years. This system, it seemed, was similar in principle to those used on space habitats. Carbon dioxide was removed by a combination of chemical scrubbers and green plants. Both of these required energy: the scrubber chemical had to be heated to drive off the CO 2 it had absorbed, and the plants required light. Since they were cut off from the sun, they got their energy geothermally, using works that Rufus and the others of his generation had sunk deep into the roots of the mountain. The maintenance of this system had been the full-time occupation of everyone in the Hole for the entire time they’d been down there. When they had neared the end of their stock of light-emitting diodes they had revived the art of making lightbulbs, consulting the Cyc for particulars, blowing artisanal glass envelopes and winding the filaments by hand. Likewise with many other things they had found themselves in need of.