Images of the surface of Mercury obtained by Mariner 10 showed a planet covered with craters, looking very much like the Earth's Moon. During its three passes by the planet, Mariner 10 took pictures of about half the surface of the planet, so we don't know what the rest of the surface looks like. Mercury's impact craters were made early in the evolution of the solar system, nearly 4 billion years ago, by meteorites which hit the surface. The relatively smooth plains between craters indicates that at one point the surface was probably volcanic, as lava flows filled in after the impacts of large objects. It appears that these plains and heavily cratered regions are not uniformly scattered around the surface. Instead, craters are concentrated on one part of the planet while plains dominate another part. Wrinkle ridges or "rupes", a few km across and hundreds of km long, are also common on the surface. These features, which can reach several km in height, were probably created as the interior of the planet cooled and shrunk, causing the surface to collapse in and form ridges. Major surface features include ridges, plains, and numerous large craters, the largest (about 800 mi or 1300 km across) named the Caloris Basin.