Bill froze for a moment, then turned to Garfield. "Earth has, or had anyway, about 330 million cubic miles of water. The river in the topopolis averages maybe a half-mile wide and a tenth-mile deep, so if it was straight you’d probably only need about fifty million cubic miles to fill it. Soil can also be up to twenty-five percent water, so depending on how deep they’ve layered real soil over the shell’s inner surface, maybe you add another hundred million cubic miles. It’s still less than what Earth had. And there’s always the Oort cloud if you needed more."