A lake is a terrain feature, a body of liquid on the surface of a world that is localized to the bottom of basin and moves slowly if it moves at all. Another definition is, a body of fresh or salt water of considerable size that is surrounded by land. On Earth, a body of water is considered a lake when it is inland, not part of the ocean, is larger and deeper than a pond, and is fed by a river. The only world other than Earth known to harbor lakes is Titan, Saturn's largest moon, which has lakes of ethane, most likely mixed with methane. It is not known if Titan's lakes are fed by rivers, though Titan's surface is carved by numerous river beds.[cissp]