Mare
Also called: maria, lunar seas.
One of the Moon's dark, smooth plains — ancient lava flows, once mistaken for seas.
The dark patches that make the “man in the Moon” are maria (singular mare, Latin for sea): vast basins flooded by dark basalt lava more than three billion years ago. The bright regions between them are older, cratered highlands.
Binoculars show them easily; a telescope reveals wrinkle ridges, rilles (collapsed lava channels) and the smaller craters that pepper them.