During their nighttime escape over the mountains, they’d had little time for such conversations, and so their knowledge of Digger culture was still spotty. Some reasonable guesses could be made simply from the known history of the Hard Rain. The phase known as the Cooling Off had not begun until some thirty-nine hundred years after Zero, when the human races’ efforts to police the lunar rubble belt had finally paid off with a sharp falloff in the number of bolides striking the surface. Until then, the Diggers had been obliged to maintain a small, steady population in the space that Rufus had provided. Expansion of the Hole had been limited by the fact that it was a sealed system, with no place to put spoil—the quantities of loose material created by digging. As anyone knew who had ever dug a hole in the ground with a shovel, the size of the dirt pile—the spoil—was always larger than the volume of the hole. They’d been able to dump some spoil down a deep and otherwise useless shaft, but once this had been filled, they’d been unable to expand their living space for as long as the Hard Rain had made a direct connection to the outside too dangerous to be contemplated. So during that phase—well over three thousand years—they had devoted all of their energies to maintaining a community of several hundred people. Hence the rigid controls on breeding. Thanks to their Cycs, they knew everything about contraception, but they had no ability to manufacture things like condoms and pills, so that lore was mostly useless. The limitations on breeding were enforced by moral strictures, by segregation of the sexes, and by surgical sterilization. This, like all of their surgery, was performed without chemical anesthesia once they’d run out of drugs, which occurred fairly soon after Zero. Apparently they had gotten rather good at acupuncture and at biting down on things.
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 CO2 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.
Ty, not really an expert on technology, made little headway in trying to imagine the particulars. Someone of a more technical bent might have devoted weeks to debriefing Sonar Taxlaw and extracting every last detail about how they had managed to get along with just the stuff available to them underground. More important for present purposes was to get a general understanding of the Diggers’ culture, and why they behaved as they did.