Smooth “maria"

"While the Apollo program and robotic landers provided a wealth of information about the Moon, these missions visited only a small number of sites. Much of our
understanding of geophysical processes and surface composition, and how these vary across the Moon, comes from Earth-based and orbital remote sensing
observations. Radar mapping of the Moon is particularly useful for probing to depths of several meters into the lunar “regolith” — a layer of packed fine dust and rocky debris that covers virtually the entire surface.
...
The Moon is characterized by two major terrain types: the old, rugged highlands, and the younger, smooth “maria.” The highlands are covered by overlapping layers of debris from basin-forming impacts. The maria are deep stacks of lava flows that flooded large impact basins, and are now covered by a thinner regolith created by small meteorite impacts. The geology of the Moon is important as it reveals a record of early solar-system history (erased on Earth by erosion), the role of impacts in shaping surfaces, and volcanic processes on a body intermediate in size between the terrestrial planets and asteroids. With the renewed emphasis on human planetary exploration, mapping of potential lunar resources such as iron, titanium, and volatiles has also gained new importance. "

Lunar Map Catalog--->

[NRAO News 99, Bruce A. Campbell (Smithsonian Institution)- Donald B. Campbell (Cornell University)]
[Acuarela. Alma Project, 2007. Connie Mendoza]