Earth's unusually large, close moon is pretty key to life on the planet, governing the regular tide cycles that may have been necessary to set the stage for the first life in the planet's oceans, but its origin has been the subject of a number of competing theories. The leading explanation
also happens to be the most awesome one -- that a planetoid about the size of Mars smashed into the Earth
some four and a half billion years ago, dashing off a chunk of the still developing planet
that hung around as the natural satellite we know and love. That theory moved a long way toward validation today with the finding that Moon rocks show chemical signatures consistent with being exposed to an explosion that was literally Earth-shattering.