A 1,200-mile-wide crater on the far side of the moon was formed when a huge asteroid with a heavy metal core smashed into the lunar surface billions of years ago. When that happened, the asteroid drilled through layers of the moon’s crust while losing mass of its own. Molten rock partially refilled the impact area, melting chunks of the asteroid’s busted metal core along the way. Peter James, a planetary scientist at Baylor University explains that today, metal from the asteroid’s core could still be embedded in the lunar mantle, causing the extra mass.
The exact size of the asteroid and the date of impact have yet to be determined but one thing is certain: Had this rock missed the moon and smacked into Earth it would have been a very, very bad day for whatever critters were living here then.