Almost 50 years ago, researchers found two massive blobs of iron-rich rock, each the size of a continent, near the center of Earth. Now, researchers have determined that these blobs—one underneath Africa and the other below the Pacific Ocean and called large low-velocity provinces (LLVPs)—are remnants of a planet called Theia that smashed into Earth 4.5 billion years ago. It was this massive collision that's thought to have formed the moon.
From Arizona State University:
It has been hypothesized that the moon was created in the aftermath of a giant impact between Earth and a smaller planet dubbed Theia. Still, no trace of Theia has ever been discovered in the asteroid belt or meteorites.
This new study suggests that most of Theia was absorbed into the young Earth, forming the LLVP blobs, while residual debris from the impact coalesced into the moon.
"The moon appears to have materials within it representative of both the pre-impact Earth and Theia, but it was thought that any remnants of Theia in the Earth would have been 'erased' and homogenized by billions of years of dynamics (e.g., mantle convection) within the Earth," said ,Steven Desch, professor at ASU's School of Earth and Space Exploration. "This is the first study to make the case that distinct 'pieces' of Theia still reside within the Earth, at its core-mantle boundary."