Lunar maria

The lunar maria (singular: ) are large basaltic plains, dark, in Earth's Moon, formed by ancient volcanic eruptions. They were called 'mare' (English: 'seas') by early astronomers who mistook these regions with terrestrial seas water because they used primitive telescopes.

They are the dark areas known as Maria, which make up about 16 percent of the surface area. Landforms on the maria tend to be smaller than those of the highlands. The small size of mare features relates to the scale of the processes that formed them -- volcanic eruptions and crustal deformation, rather than large impacts. The chief landforms on the maria include wrinkle ridges and rilles and other volcanic features. Wrinkle ridges are blisterlike humps that wind across the surface of almost all maria. The ridges are actually broad folds in the rocks, created by compression. Many wrinkle ridges are roughly circular, aligned with small peaks that stick up through the maria and outlining interior rings. Circular ridge systems also outline buried features, such as rims of craters that existed before the maria formed.

When the far side of the Moon was first photographed by the Soviet Luna 3 probe in 1959, it was observed an almost complete absence of lunar maria.