The smooth dark areas on the Moon's surface are called maria (plural for mare; Latin for seas). These volcanic plains are made up of a rock type known as basalt, similar in composition to the rocks found in Hawaii. They cover 17% of the surface area of the Moon. The maria contain physical features such as pits and channels, but lack large volcanos.
Features visible in this image of the western part of the lunar nearside include Mare Imbrium, Mare Humorum, Mare Nubium, and the craters Copernicus and Kepler. Copernicus is the most prominent crater with its bright rays composed of ejecta material.
This image was taken by one of NASA's Lunar Orbiters in 1967.
Image Credit: NASA
Credit: Lunar and Planetary Institute