Our moon was likely formed around 4 billion years ago when a planet, approximately the size of Mars, collided with a newly forming Earth. The resultant debris coalesced (or came together) forming the Moon. However, a new theory suggests Earth once had two moons. The new theory incorporates the "global impact theory" but tries to explain the unusual geographical features of our moon - as it currently exists. When we look up at the moon we always see the same side - a relatively flat and heavily cratered surface - while the far side of the moon is very different - with huge mountain ranges towering 3km towards the edge of space. This observation baffled scientists for years but the new theory tries to explain the moon's unusual split personality. Put forward by two researchers, Erik Asphaug & Martin Jutzi from the University of California Santa Cruz, the theory suggests the mountainous side of the moon was caused by a smaller satellite, roughly one thirtieth of the mass of the existing moon, colliding slowly with its larger cousin. The scientists used computer simulations of an impact between the Moon and a smaller companion to study the dynamics of the collision and track the evolution and distribution of lunar material in its aftermath. The results were consistent with the Moon's current geology. For more information on this new theory visit the BBC Science website.