The theory that the Moon was created by the impact of a Mars sized body with the Earth was given a bit of a boost today as research from the data archive of the Japanese Kaguya probe identified outcrops of the mineral olivine surrounding deep craters.
In the collision theory, one of the key predictions would be the production of massive amounts of olivine inside the new body. The trouble is, this stuff is not that dense relative to other stuff crystalising out of the body and so at some point, the mantle would have to churn over and leave the stuff just below the crust. In this case, large impacts would have the effect of excavating the rock and leaving traces on the surface for later probes to find. Kaguya found the mineral lying in concentric circles at 34 sites, three of which were already known.