Material deep inside Earth -- thousands of kilometres down, near the planet's core -- has undergone a mysterious shift.

Although the change occurred nearly two decades ago, between 2006 and 2008, scientists discovered it only recently, while analysing data from a pair of satellites that once measured variations in Earth's gravity. The team thinks it might have happened when the structure of some of the rocks near the boundary between Earth's core and mantle transformed, becoming denser.

The discovery -- possible because the geological shift altered the planet's gravitational field -- is an astonishing testament to Earth-orbiting satellites. "It's a really new observation," says Isabelle Panet, a geophysicist at the University Gustave Eiffel in Paris. Along with lead author Charlotte Gaugne Gouranton at Paris City University and other colleagues, Panet reported the findings last month in Geophysical Research Letters.

The work will help scientists to better understand the connections between Earth's various layers, from its brittle crust to its solid mantle to its partially liquid core, Panet says. Connections between these layers affect where large earthquakes originate, how the planet maintains a magnetic field that protects it from solar storms and more.

Panet's team made the discovery using data from a pair of US-German satellites known as the Gravity Recovery and Climate Experiment (GRACE), which orbited Earth between 2002 and 2017. The satellites flew one in front of the other, separated by a set distance.

When they encountered a gravitational tug from, for example, the hulking mass of a mountain range, the lead satellite would pull temporarily away from the trailing satellite -- a change that could be measured and correlated with the gravitational shift. Researchers have most often used changes in the distance between the GRACE satellites to measure the displacement of masses of water on Earth, such as when groundwater disappears beneath croplands or when glaciers melt.

But, as it turns out, GRACE was also able to spot much deeper changes in Earth's mass. Panet had already used it to look for mass changes happening hundreds of kilometres below the surface ahead of large earthquakes. Then she realized she could probe even farther down, to a depth of nearly 2,900 kilometres, to the complex boundary between the core and mantle.

Panet and her colleagues spotted a strange signal in the GRACE data that peaked around 2007 and was centred off Africa's Atlantic coast. They were unable to attribute it to water shifting around on Earth's surface, however. "So at least partially, there has to be an origin within the solid Earth," Panet says. "It has to come from very deep."

Using satellites that measure changes in Earth's magnetic field, scientists had previously detected some geomagnetic perturbations that occurred in the same region around 2007. Panet thinks that the latest gravity observations might tie in with that observation to tell a coherent story.

In this scenario, a type of mineral called a perovskite, in rocks near the bottom of Earth's mantle, would have changed its structural configuration in response to the crushing pressures deep inside the planet. This mineral change, in turn, would have caused the rocks in this region to become denser and increase in mass.

Neighbouring rocks would then have shifted position slightly. Similar shifts would have rippled outwards and, ultimately, might have reached the core-mantle boundary, which would have responded by deforming, perhaps on the order of 10 centimetres. That small change in the boundary could have affected the liquid flowing in the underlying core, in ways that would have shown up as the weird blip in magnetism.

It's a complex story that might or might not pan out after more study, Panet cautions: "It still has to be further investigated."

Nevertheless, the discovery is "very intriguing", says Barbara Romanowicz, a seismologist at the University of California, Berkeley. "For the first time," she says, "we have convincing evidence of dynamic processes at the base of the mantle" that are occurring quickly enough to study "as they happen".

Panet and her colleagues have not spotted any big gravity signals in the satellite data other than the one that happened in 2007. But she plans to monitor measurements coming from GRACE's successor satellites, which are currently in orbit, to see whether any other mysterious changes are afoot deep inside Earth.