Mare Crisium surrounded
by cratered higlands.
| If you look at the surface
of the Moon, you'll see that it can be divided broadly into two
types of terrain: Dark flat plains and bright-colored heavily cratered
highlands. The dark plains, called maria (the Latin word for seas),
are where the basalts come from. Early in the Moon's history it
was bombarded by giant meteorites that formed huge circular impact
basins. Quite a long time later, lavas came up through the cracks
in the lunar crust and filled these basins, creating the maria.