Did our planet once have two moons? Some researchers say so. Moreover,
the missing satellite might still be up there splattered across the far
side of the Moon. NASA's GRAIL mission could help confirm or refute the
"two moon" hypothesis.
This Video from Nasa is a must watch and to be honest, it is a very interesting Theory. I was under the impression that the far side of the moon faces the asteroid
belt. The moon then acts like a shield, somewhat protecting the Earth
from impending asteroid attacks. The reason why one side is so smooth,
and the other is so mountainous is because the "far side" gets pummeled
by asteroids. The "mountains" are really craters. We never see the far
side of the moon because it moves around the Earth at the same pace it
revolves on it's axis. Therefore, the far side will always be hidden.