Incredible Unsolved Mysteries

Researchers gazing into Earth's interior have discovered two continent-sized structures that turn our view of the mantle on its head. What does their existence mean for our return to the Earth's surface?

Seismic tomography images show a portion of a "speck" at the base of the mantle beneath Africa. The slow-wave velocity region above the speck, including the tip and branches, may indicate a plume or upwelling.

About 2,000 km below our feet, there is a large amount of hot mantle material that has been troubling scientists for the past four years.

As some scientists claim, these patches are the length of the continent, 100 times higher than Mount Everest. They are located at the bottom of the Earth's rocky mantle above the molten outer core, a place so deep that the planet's elements are squeezed out of existence. These spots are made of rock, just like the rest of the mantle, but they may be hotter, and heavier, and hold the key to unlocking the story of Earth's past.

Scientists first discovered these spots in the late 1970s. Researchers have just invented a new way to look inside the Earth: seismic tomography. When an earthquake shakes the Earth, it releases waves of energy in all directions. Scientists track these waves as they reach the surface and calculate where they are coming from. By looking at the travel times of many seismic waves from thousands of instruments around the world, scientists can reverse-engineer a picture of the Earth's interior. The process is similar to a doctor using ultrasound equipment to image a fetus in the womb.

"Ultimately, many people think plate tectonics is one of the reasons we have life on Earth," said Harriet Lau, a Harvard geophysicist. Scientists believe these patches play a role in many processes deep within the Earth, including plate tectonics and volcanism.

Once researchers began to form images of Earth's interior, they began to see things they never imagined.

"From the beginning, these models made it very clear that at the bottom of the mantle, halfway down near the center, there are these huge regions where waves travel more slowly," said Edganello, professor of Earth and space exploration At Arizona State University.

The slow-wave velocity zones are concentrated in two locations: one beneath the Pacific Ocean and the other beneath Africa and part of the Atlantic Ocean. They look like "big mountains on the core-mantle boundary," says Sanne Cottaar, a seismologist at Cambridge University. Other researchers have described them as "overlapping" conical gravel craters or giant piles of sand. The spots are so large that if they were on Earth's surface, the ISS would need to navigate around them.

"They're unmissable," says Karin Sigloch, a seismologist at Oxford University. "They just appear in everyone's photos."

There's no doubt the spots exist, but scientists don't know what they are. A recent paper says the spots "remain mysterious. Scientists can't even decide what to call them. They go by many names, the most common being LLSVP, which stands for Large Low Shear Velocity Province.

Part of the mystery is what Earth scientists have been grappling with: they can never visit the interior of the planet. "We know less about the depths beneath our feet than we do about the Sun, the Moon, or the surface of Mars," says Paula Koelemeijer, a researcher at University College London. Scientists have been trying to come up with new ways to indirectly peer into the Earth's interior.

Fortunately, technological advances in sensing tiny oscillations inside the Earth, as well as efforts to equip more sites with instruments, have been moving the field forward. Several recent studies of cutting-edge technology are bringing new insights.