A new theory suggests the Earth once had a small second moon that perished in a slow motion collision with its "big sister".
Researchers suggest the collision may explain the mysterious mountains on the far side of our Moon.
The scientists say the relatively slow speed of the crash was crucial in adding material to the rarely-seen lunar hemisphere.
Details have been published in the journal Nature.
The researchers involved hope that data from two US space agency (Nasa) lunar missions will substantiate or challenge their theory within the next year.
For decades, scientists have been trying to understand why the near side of the Moon - the one visible from Earth - is flat and cratered while the rarely-seen far side is heavily cratered and has mountain ranges higher than 3,000m.
Various theories have been proposed to explain what's termed the lunar dichotomy. One suggests that tidal heating, caused by the pull of the Earth on the ocean of liquid rock that once flowed under the lunar crust, may have been the cause.
But this latest paper proposes a different solution: a long-term series of cosmic collisions.
The researchers argue that the Earth was struck about four billion years ago by another planet about the size of Mars. This is known as the global-impact hypothesis. The resulting debris eventually coalesced to form our Moon.
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