The far side of the moon looks very different than the near side: There are only a few small maria. Data from the manned moon missions suggests that the lava flows are caused by radioactive heating. In that case, the fact that most of the maria are on the near side is a coincidence. Sometimes this is called the dark side of the moon, but it is actually a little lighter.
Credit: NASA/JPL/USGS/The Clementine Project
