There’s a weird blob of something in the moon’s innards,and even scientists are not completely certain how it got there.
On the far side of the moon, buried nearly two hundred miles under the South Pole-Aitken basin (the largest preserved crater in our solar system), and is a mysterious mass. Peter B. James and his team of scientists from Baylor University believe it could be the metal core of an asteroid that head-butted the moon and left that 1242-mile-wide crater behind.
Source: blastr.com