A quadrillion tons of diamond may be hiding beneath your feet. But before you grab your supervillain cape and nab the nearest mega-drill, be warned: At around a hundred miles deep, these riches lie well out of reach.
Scientists found the astonishing quantity of sparkly gems using seismic waves rippling through Earth to estimate the composition of a particular planetary layer. Their results, published recently in the journal Geochemistry, Geophysics, Geosystems, show that our planet is probably packed with a thousand times more diamonds than researchers previously thought.
Organizations like the U.S. Geological Survey use an array of sensitive instruments known as seismometers around the world to monitor each tremble and burp our planet makes. Not only can this info help scientists better understand temblors, but researchers can also use it to create pictures of Earth's innards. The stiffness, temperature, density, and composition of the rock all affect how these waves travels through it. So by examining our planet's shivers, researchers can estimate what hides inside—no mega-drills needed.
Over the years, however, researchers have noticed something odd. Around a hundred miles below the surface, seismic waves move much faster than expected. This region holds what's known as cratonic roots—inverted mountains of "cold, stable, stiff mantle that's supporting the continents above them," says Joshua Garber, a postdoctoral researcher at Penn State University and author of the study.
Cratonic rocks are strikingly old, forming and stabilizing in the first two billion years of Earth's 4.5-billion-year history. Researchers have gotten hints to their composition through rare volcanic eruptions that send deep pools of magma rising toward the surface, carrying chunks of the ancient rock. But the exact composition of cratonic roots—and the reason sound waves zip through them—had long been a mystery. (For instance, scientists recently found a previously unknown type of mineral in a super-deep diamond.)
In the new study, researchers used seismic data to study different models that simulate the three-dimensional warbling of waves moving through Earth. They then considered rock composition, buoyancy, and even electromagnetism of various possible materials to tweak the recipe of the rocks that create the observed wave speeds.
"What we found, in contrast to previous studies, is you can't just use the main rock type in the mantle—that's called peridotite—to explain these velocities," says Garber. "You need something that's a little stiffer."
The answer: diamonds, and lots of them.
Combined with a dash of oceanic crust rock called eclogite, just over a quadrillion—a one with 15 zeroes—tons of diamonds are hiding within Earth, they estimate. To be fair, that's only a small percentage of the total rocks on Earth, Garber notes. By volume, around 2 percent of the cratonic roots are made of diamond.
Researchers have long known that a massive amount of carbon swirls beneath Earth's crust as carbon dioxide and carbon-rich minerals such as graphite, calcite, and diamond. Though these sparkly gems can fetch a fancy fee at a jewelry store, geologically speaking, they're not exactly rare.
“It was unexpected, but it's not unprecedented” Garber says of the newest diamond estimate.
Even so, a quadrillion tons is a lot of sparkle. "There is absolutely something that speaks to our imagination here," says Maureen Long, a seismologist at Yale University who wasn't involved in the work.
Long adds that the study is “an exciting and elegant result in a lot of ways,” and she emphasizes that the research is carefully done and accounts for many influencing factors.
But it may not be the last word on Earth's diamond stash: “I'm taking the conclusions, personally, with a grain of salt,” says Suzan van der Lee, a seismologist at Northwestern University who wasn't involved in the research. Though she praises the meticulousness of the work, there are other models yet untested that might provide a different result.
For now, both Long and van der Lee agree that the work is a step in the right direction, since it comes from a large group of collaborators with many types of background and expertise. Also known as Cooperative Institute for Dynamic Earth Research, this month-long affair brings together all manner of scientists for focused research.
And one important question remains: Will humans ever be able to get at these diamond riches? It's unlikely, says Garber. “At least, [not] on human timescales,” he adds. So far, even using cutting-edge technology, humans haven't dug more than 7.6 miles deep, so making a hole of hundred miles or more would certainly be a supervillain-size challenge.