Neither case is settled. But in recent years, many earth scientists have begun to make the case that these vague shapes are piles of dense, smoldering rock that date to the dawn of the planet. And multiple studies in the past year have argued that their persistent influence might be responsible for long-puzzling patterns in volcanic hot spots such as Hawaii.

“These are the largest things on the planet,” says Ed Garnero, a seismologist at Arizona State University. “Only recently have I started thinking, ‘Wow, this is potentially super profound.’”

If an omnipotent scientific illustrator halved the Earth, he or she would first need to cut through the thin crust we live on, which is broken into shifting tectonic plates. Then would come the rocky mantle. Only at 2,900 kilometers down, about halfway to the very center, would he or she hit the core-mantle boundary.

To map that part of the Earth, seismologists use the waves released by earthquakes. As the waves rattle outward, they change speed depending on what material they pass through. That causes them to arrive at different monitoring stations at different times. In 1984, the Harvard researcher Adam Dziewonski first integrated data from many different earthquakes into a global map. The two blobs showed up immediately, attached to the core on either side like Princess Leia’s side buns.

In these regions, earthquake waves seem to slow down, suggesting that the blobs are hotter than the surrounding mantle. How do we know this? Rock expands when heated. That causes waves to travel sluggishly through warm regions, Garnero says, like vibrations moving through a loose guitar string.

The slowing waves gave these features their formal name: large low-shear-velocity provinces, or LLSVPs—an unmagical initialism that may have contributed to the topic’s low profile. “We are also to blame,” as Sanne Cottaar, a seismologist at the University of Cambridge, put it, “for misnaming this feature so badly.”

At first, earth scientists contemplating these warm patches argued that their one obvious trait, the warmth, was where the story ended. Some still do.

This school of thought holds that the blobs are mostly just thermal features. Over time, the mantle roils like an unbearably slow-boiling pot of water. Heat comes from the bottom, where the mantle touches the core, and this heat causes rock in the mantle to waft up in plumes. Where seismologists map blobs, they could just be seeing the blurred bases of the world’s biggest clusters of hot plumes.

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In this view, the blobs are mostly made up of the same stuff as the rest of the mantle. And their placement is dictated by plate tectonics from above, not by anything inherent and spooky about these regions. When one tectonic plate in Earth’s crust is pushed below another in a process called subduction, it sinks. This sends colder rock down into the mantle.