The large impact hypothesis is currently the favored explanation for how the Moon was formed. It proposes that a Mars-sized object collided with the early Earth, sending debris into orbit that eventually coalesced to form the Moon. When first formed 4.5 billion years ago, the Moon's surface was molten and covered in lava oceans that cooled to form large dark basins called maria. The lighter colored highlands are made of the rock anorthosite and were bombarded by meteorites, creating many impact craters visible today. While no life exists now on the airless, water-poor Moon, recent discoveries have found frozen water trapped at its poles.