Most astronomers now believe that, 4.5 billion years ago, a planet the size of Mars collided with the young Earth. Some of the debris from this cataclysm went into orbit around the Earth and collected together under its own gravity to form the Moon.

The effect of this impact would have been absolutely devastating to the Earth. The entire crust of the planet would probably have been blasted off into space, leaving behind an Earth whose entire surface was an ocean of molten magma.