Pieces of Theia live inside Earth.

According to an article in Science News, researchers propose that remnants of a Mars-sized object called Theia, which collided with Earth about 4.5 billion years ago and formed the moon, still exist today as dense material on Earth's core. Geophysicists have discovered two large zones of rock at the base of Earth's mantle, known as large low-velocity provinces, which are denser than the rest of the mantle rock. While some scientists believe these masses are remnants of tectonic plates, the researchers suggest that they are actually remnants of Theia's core and mantle. The simulations conducted by the researchers indicate that a substantial portion of Theia's core and mantle would have sunk into Earth's core and uppermost mantle, respectively, after the collision. The team proposes that the material from Theia's mantle slowly sank over millions of years to form the large low-velocity provinces. The article also mentions that a collision between Earth and a protoplanet is the leading theory for the moon's formation and may have also jump-started plate tectonics. [Extracted from the article]