Dense spots buried deep beneath the surface could be real pieces of an ancient planet.

In new research, scientists speculate that Earth was formed by a Mars-sized rogue planet known as Theia, and identifiable fragments of the alien planet are still found deep inside Earth.

Scientists believe the same fateful impact also formed the moon, but this is the first time they have studied the specific traces of the crash left in Earth’s mantle.

In 2016, researchers at UCLA proposed that Earth could actually be two planets that merged after colliding: itself and Theia. At the time, scientists said they believed the two planetary masses were mixed evenly. Now, Qian Yuan of Arizona State University and her colleagues suggest that the mysterious dense spots in the Earth’s interior are specific pieces of Theia that are still intact.

Yuan’s team presented a paper earlier this month at the 52nd Lunar and Planetary Science Conference. In it they explain:

“Here we demonstrate that Theia’s mantle may be several percent intrinsically denser than Earth’s mantle, allowing materials from Theia’s mantle to sink to Earth’s lower mantle and accumulate in thermochemical stacks that can cause the [dense areas] observed seismically.”

These dense areas are “continent-sized high low shear velocity provinces (LLSVPs).

Arizona State scientists identified specific portions of the Earth’s interior and modeled the process by which these portions could have struck the early Earth and plunged into its center:

“We define successful models in which [Theia’s mantle materials] sink to the lower mantle and form in spatially isolated thermochemical stacks that occupy 3 to 15% of the 2D model domain area, similar to current LLSVPs that occupy ~3-9% by volume of the Earth’s mantle.”

In an interview with Vice, Yuan talked about how he was inspired after chatting with a colleague about LLSVPs. “When I got back to my office,” Yuan said, “I just did a very simple calculation where I compared the size of the LLSVPs to the mantle of Mars, because Theia was believed to be very similar to Mars. The total mass of the Moon, along with the LLSVPs, almost perfectly matches the mantle [of Mars].”

How is it possible that Theia’s dense materials have remained intact for billions of years? It is a function of the way the Earth’s mantle works, where materials that have a certain temperature and density circulate through convection. Theia’s materials are so dense that they sank and never floated back to the convection zone. Think of it like stuff that builds up in a sharp corner that’s hard to reach with a vacuum cleaner.

And what does it mean that these materials remain in the depths of the Earth? “Continental-sized” areas are the largest portions of Earth’s interior, so understanding how they formed and how they persist could help advance Earth science. These two huge chunks lie beneath West Africa and the Pacific Ocean and are hundreds of kilometers thick, HuffPost explains.

There could also be interesting parallels to be discovered, so to speak, between these dense portions of the mantle and the Moon due to their potentially shared origin history. How often can you compare a 4.5 billion-year-old rock sample orbiting in space and one sunken to the center of the Earth? It is the dream of the scientific method come true.