Building Harlequin's Moon

by Larry Niven

went cold again. Then we brought in more comets. The comets gave us the water you see here.” “So the water is from space?” Rachel asked. “Isn’t everything? This deep sea is the motor that drives Selene’s hydrology. We need this much water for the humidity to grow tropical plants rather than cactus. But we had to contain most of the water to limit the effect of the tides, which are worse here than on Earth, because of Harlequin.” He pointed at the gas giant, which hung just off center in the sky. “Gravity pulls the water toward Harlequin. If we didn’t contain it, Selene would be flooded with every high tide. We could have made hundreds of small seas instead of what we did make; the Hammered Sea and Erika’s Folly, but they would have been harder to manage. Oh, lots of craters have a little water in them, but over half our water is right here.” He paused, and said, “Besides, I wanted to make a real sea.” That made him feel