Mare Imbrium

The Sea of Rains fills a basin gouged out roughly 3.9 billion years ago, then flooded by basalt over the following few hundred million years. At 1,146 km across it is the second-largest mare on the Moon and by far the most sharply defined, ringed by the Apennine, Caucasus and Alpine mountains.

Because the flooding came long after the impact, the surface is smooth and relatively crater-poor. Wrinkle ridges cross it where the cooling lava settled and buckled — best seen when the terminator lies nearby.

Apollo 15 landed at the basin's southeastern edge in 1971, near Hadley Rille, sampling both the mare basalts and the older highland material thrown up by the impact.