Lunar mare
The Lunar maria (singular: mare, pronounced MAH-ray) are large, dark, basaltic plains on Earth's Moon, formed by ancient basaltic flood eruptions caused by extremely large meteoroid impacts. They were dubbed maria, Latin for "seas", by early astronomers who mistook them for actual seas. The maria cover 16% of the lunar surface.
Most of the volcanic eruptions that produced the maria occurred at many points along fissures around the edges of the impact basins. The basaltic lava usually flowed out in several distinct episodes over the course of many thousands of years, well after the initial formation of the basins themselves. Although these are relatively flat surfaces, during their contraction phase they can produce low, blister-like formations known as wrinkle-ridges
Most of the maria are on the near side of the Moon (the side facing the Earth). The far side has relatively few maria.
The traditional nomenclature for the Moon also includes one "oceanus" (ocean), as well as features with the names "lacus" (lake), "palus" (marsh) and "sinus" (bay). The latter three are smaller than maria, but have the same nature and characteristics.
See also
ja:月の海