Decades ago, scientists first harnessed the echoes of earthquakes to make a map of Earth’s deep interior. They didn’t just find the onion layers you might remember from a grade school textbook — core and mantle covered by a cracked crust. Instead, they saw the vague outlines of two vast anomalies, unknown forms staring back from the abyss.

Over the years, better maps kept showing the same bloblike features. One huddles under Africa; the other is beneath the Pacific. They lurk where the planet’s molten iron core meets its rocky mantle, floating like mega-continents in the underworld. Their highest points may measure over 100 times the height of Everest. And if you somehow brought them to the surface, God forbid, they contain enough material to cover the entire globe in a lava lake roughly 100 kilometers deep.

“It would be like having an object in the sky, and asking, ‘Is that the moon?’ And people are like, no. ‘Is that the sun?’ No. ‘What is it?’ We don’t know!” said Vedran Lekić, a seismologist at the University of Maryland. “And whatever it is, it is intimately tied to the evolution of the Earth.”

The first mystery of these hulking, hidden seismic features is whether they’re made of different stuff than the rest of the Earth’s mantle. The second: How do these patterns in the deep leave traces on our surface world?

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 like Hawaii.

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

Core of the Matter

If an omnipotent scientific illustrator halved the Earth, they would first need to cut through the thin crust we live on, which is broken into shifting tectonic plates. Then they’d pass through the rocky mantle. Only at 2,900 kilometers down, about halfway to the very center, would they 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 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, said Garnero, like the slower 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 abbreviation that may have contributed to the topic’s low profile. “We are also to blame,” said Sanne Cottaar, a seismologist at the University of Cambridge, “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 base regions of the world’s biggest clusters of hot plumes.

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.

Yet no plates have subducted over the blob regions for the last few hundred million years, said Saskia Goes of Imperial College London. “It’s the absence of cold material that makes these relatively hot.”

The opposing camp, meanwhile, doesn’t doubt that plumes rise from the hot blob regions. They just argue that the blobs are special in and of themselves.

Since the mid-2000s, several teams of seismologists have looked at earthquake signals that just graze the edges of these regions. Those signals show complicated patterns, indicating that the waves were skimming across a relatively crisp boundary. This suggests that the edges of the blobs mark a transition between materials, not just temperature.

In this view, the blobs are so-called thermochemical piles, clumps of dense rock with a distinct chemical composition. Because of their prolonged contact with the core, they are hotter than the rest of the mantle, causing plumes to sprout.

Assuming that the blobs are distinct, they could be old — the last surviving remnants of the infant Earth. One leading idea is that they formed when the entire lower mantle was an ocean of magma, shortly after the planet’s birth. Rock began to cool and crystallize, but iron stayed melted in the magma ocean, said Nicolas Coltice at the École Normale Supérieure in Paris. Then, when the last dregs of magma crystallized, they were so dense and iron-rich that they sank to the bottom of the mantle, forming the blobs.

Down there, they would have held out through the early planet’s greatest cataclysm: a hypothesized impact with a Mars-size body called Theia that ultimately birthed the moon. Or, Garnero speculates, the dense, distinct piles might even be fragments of Theia itself, forever interred in the deep Earth.

In the thermal-only view, plate tectonics are the true movers and shakers of the world, dictating where upwelling happens. But the thermochemical-piles camp believes that hot, heavy, stable blobs would have more of a back-and-forth dialogue with the tectonic system on the surface. Cold currents from sinking plates would push the blobs around like Silly Putty; in turn, upwelling heat from the warm blobs would push the plates right back.

Hot Spot Puzzles

To test how much the blobs are helping to pilot the geophysical ship, scientists looked to Hawaii. In the past year, researchers invoked the blobs to solve two long-standing puzzles there.

Consider first the Hawaiian-Emperor chain, a stretch of islands and underwater mountains. The chain starts at the still-growing Big Island and spans 6,200 kilometers, ending near Russia. Geologists have long explained the chain as a hot spot: As the Pacific plate slides over a fixed mantle plume, the plume pushes up new volcanic islands from below.

The only trouble is the bend. Smack in the middle of the chain is a 60-degree kink. The bend, geophysicists thought, came from a long-ago shift in the plate’s motion.