On September 10, 2011 NASA launched the Gravity Recovery and Interior Laboratory (GRAIL) satellites on a mission to the Moon. GRAIL-A and GRAIL-B are nearly identical spacecraft, except that B is designed to follow A around the Moon in the same orbit. The Lunar Gravity Ranging System will measure the distance between the two spacecraft, watching for minute deflections caused by anomalous mass concentrations or mass deficits beneath the Moon's surface.
In the image at the top of the page, anomalous areas of increased and decreased expectations were mapped by the Lunar Prospector in 1998-1999. Anything in yellow indicates what computer models of the Moon predicted. Red and purple mean that there is a higher gravity field than expected, while blue and green indicate a lower field. On the left, red concentrations that do not correspond to simulations do correspond to the great maria, or "seas" on the Moon. The five largest are Mare Imbrium, Mare Serenitatus, Mare Crisium, Mare Humorum and Mare Nectaris. On the right, or the farside of the Moon, circular areas of lower gravity can be seen.
There is a major elevation difference between the two hemispheres, as well. The nearside of the Moon is flat, with vast maria, whereas the farside is dominated by mountains and is heavily cratered.