Scientists think that, for about the first billion years or so after the Moon formed, it was struck by great numbers of meteorites, which blasted out craters of all sizes. The energy of so many meteorite collisions caused the Moon’s crust to melt. Eventually, as the crust cooled, lava from under the surface rose up and filled in the larger cracks and crater basins. These younger regions, which look darker than the older, mountainous areas, are the “seas” (maria in Latin) on the Moon.