Scientists have long agreed that the Moon formed when a protoplanet, called Theia, struck Earth in its infancy some 4.5 billion years ago. Now, a team of scientists, based on new isotopic and modeling evidence, has a provocative new proposal: Theia’s remains can be found in two continent-size layers of rock buried deep in Earth’s mantle. Lava from islands tied to these deep layers, called large low-shear velocity provinces, indicates the LLSVPs may be ancient, forming from the time of the impact. And Apollo Moon rocks suggest that Theia was large, nearly the size of the early Earth, with an anomalously dense mantle. And that dense mantle lines up with models that would have allowed Theia’s mantle to sink and persist in Earth for billions of years.