The Moon features two distinct types of surface features: craters and rilles. Craters are circular depressions in the surface of a celestial body, usually caused by an impact from a meteoroid or comet. They are typically named after famous scientists or astronomers. Rilles are long, narrow valleys or trenches that are usually found on the Moon's maria, or dark, flat plains. They are believed to have been formed by volcanic activity or tectonic forces.