The Biggest Hole in the Solar System

There's a huge splotch on the southern hemisphere of the farside of the Moon. This megasmudge is South Pole-Aitken (SPA) basin, 2500 km in diameter and over 12 km deep. It is darker and richer in iron than the rest of the lunar highlands. Such immense craters formed by impact onto the lunar surface. Calculations indicate that the floor of SPA basin ought to be composed mostly of rock derived from the mantle of the Moon, but using spacecraft data Paul Lucey (University of Hawaii) and his co-workers suggest it is at most half mantle, half crust. Taking a different approach, Carle Pieters (Brown University) and her colleagues suggest that no mantle is present. Why is so little mantle present? Is our understanding of the formation of craters incomplete? Was there something unusual about the impact, such as the projectile striking the Moon at a low angle? What's going on?