Two continent-sized structures deep inside Earth may contain remnants of Theia, the ancient planet thought to have collided with Earth and helped form the Moon. Computer simulations suggest dense, iron-rich material from Theia could have survived the impact, sunk toward Earth’s core and remained there for 4.5 billion years. Chemical tests of volcanic rocks and lunar material could help determine whether these deep structures really preserve pieces of a lost planet. Deep beneath Africa and the Pacific Ocean sit two continent-sized masses unlike the mantle around them. Seismic waves slow as they cross these dense regions, and a new explanation traces their origin to the collision that formed the Moon. The structures, known as large low-velocity provinces, or LLVPs, lie near Earth’s core-mantle boundary. Each is about twice the size of the Moon. Their unusual density and seismic behavior have made their origin a long-running puzzle. A Caltech-led study published in Nature proposes that the LLVPs may contain remnants of Theia, the protoplanet thought to have struck the young Earth about 4.5 billion years ago. …