They are Seas. A long time ago when the Moon's core was still active, hot lava would rise from inside the Moon and fill these giant craters (they were called basins back before they were filled with lava). We of course call them seas today because, go figure, some kind of liquid filled them up (lava).
So they were basins before they were filled with lava and now they are called Seas because they were filled up.
rilles
maria
Dark plains
maria
Mercury looks like the Moon: it is heavily cratered with regions of smooth plains, has no natural satellites and no substantial atmosphere. However, unlike the moon, it has a large iron core.
The lunar maria are large, dark, basaltic plains, formed by ancient volcanic eruptions
maria
Dark plains
this means that there is a preferably dark spot on the moon or seas of the moon or plains on the moon
They are called lunar maria, formed by ancient volcanic eruptions that flooded the surface with basaltic lava. These smooth, dark areas contrast with the rugged, bright highlands on the moon.
moon
maria
I think you're asking about the moon's "mares," which are broad, relatively smooth plains on the lunar surface. The word "mare" is Latin for sea, which is what early astronomers mistook the plains for.
The moon has plains,mountains,and many craters. The moon also has unactive volcanoes and dark spots called seas.
Mercury looks like the Moon: it is heavily cratered with regions of smooth plains, has no natural satellites and no substantial atmosphere. However, unlike the moon, it has a large iron core.
The lunar maria are smooth and dark regions found on the moon' surface. Maria cover approximately 17 percent of the lunar surface area.
The broad flat plains on the moon are called lunar maria. They are large dark areas on the moon's surface formed by ancient volcanic activity. Mare Tranquillitatis, Mare Imbrium, and Mare Serenitatis are some well-known lunar maria.
The lunar maria are large, dark, basaltic plains, formed by ancient volcanic eruptions