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.