According to a recently published paper, scientists suspect a second moon once circled Earth in the same orbit and at roughly the same speed as our moon. It eventually bumped into its companion, but instead of causing an impact crater, the second moon stuck and made a mountain. That feature today would be the lunar highlands located on the side of the moon that permanently faces away from Earth.
MORE: http://www.msnbc.msn.com/id/45820164/ns/technology_and_science-space/#.Tv1YrPJRF8E
See also:
NASA Hopes to Answer Questions About the Moon (video)
http://video.msnbc.msn.com/nightly-news/45819973/#45819973
And...
Must-see science videos of 2011
http://cosmiclog.msnbc.msn.com/_news/2011/12/29/9805608-must-see-science-videos-of-2011
Physics World reveals its top 10 breakthroughs for 2011
http://physicsworld.com/cws/article/news/48126
2011's biggest science stories, and why they'll be back in 2012
http://arstechnica.com/science/news/2011/12/2011s-biggest-science-stories-and-why-theyll-be-back-in-2012.ars
11 scientific twists from 2011
http://cosmiclog.msnbc.msn.com/_news/2011/12/27/9748604-11-scientific-twists-from-2011
