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.’”
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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.
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