The Lunar maria (singular: mare) 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.
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.