The Two Faced Moon

The moon is "tidally locked" to Earth, meaning only one hemisphere faces us. We know that side well, with its dark regions called maria, or "seas," of cooled magma. Oddly, however, these maria are virtually absent from the back side of the moon, as has been revealed to us by probes (and seen in person by Apollo 8 astronauts). The proverbial "dark side of the moon" also is much more

 pockmarked by craters.

The starkly different hemispheres have been partly explained by the far side having a crust roughly 9 miles (15 kilometers) thicker than that of the near side. The crust on the side facing us could have more easily cracked under the onslaught of meteorites, causing maria-forming magma to be released from deeper in the moon. But that crustal asymmetry is an enigma itself