Theia

Three conflicting hypotheses began to emerge in the 19th century to explain the origin of the Moon, which had long been a mystery. In one, the primordial solar nebula's region where Earth and the Moon formed together. In a different scenario, the early Earth spun so quickly that it sent forth a glob that eventually formed the Moon. (This idea had a variant that said the Pacific Ocean was the gaping hole.) The Moon developed somewhere else in the third but was taken by Earth. None of these hypotheses, however, was able to fully describe the Earth-Moon system as it exists now. A novel theory that the Moon was created by a massive impact surfaced in the 1970s.

Theia was the name given to the impactor, which would have been the size of Mars (after the mother of Selene, the Greek Moon goddess). Theia was destroyed in the collision, and the leftover debris produced the Moon. Two moons, one smaller than the other, may have even been created by the debris. They eventually came together slowly in what has been compared to a sizable landslide. This explanation would explain why the near and far sides of the Moon are so different—the landslide site evolved into what is now the Moon's far side.