And then, there was one

Astronomers at the University of California- Santa Cruz have proposed a new explanation for the presence of the lunar highlands on a body whose general topography and geological deadness make them something of a mystery.

Some have conjectured that they might be the result of a collision with another planet, approximately the size of Mars. But the UC- Santa Cruz astronomers suggest that originally, the Earth had two or even more moons- and the highlands are the result of crashes which destroyed the others.

HT: Drudge