Now, 25 miles per year doesn't sound speedy at all, considering jet streams in the atmosphere can go as fast as 250 mph. "[B]ut you have to remember this is a very dense liquid metal," Dr. Chris Finlay from the National Space Institute at the Technical University of Denmark (DTU Space) told BBC. "[I]t takes a huge amount of energy to move this thing around and that's probably the fastest motion we have anywhere within the solid Earth."

The scientists who discovered the rapidly-moving stream of metal believes it's the result of two core layers moving towards a single boundary due to buoyancy or changes in the magnetic field within the core. When the liquids meet at that boundary, the mixture is squeezed out sideways to form the jet.

ESA's Swarm satellites measure and track the different magnetic fields caused by the Earth's core, crust, oceans, mantle and even the ionosphere and the magnetosphere. By monitoring magnetic fields, the satellites give scientists a way to figure out how the core's layers move -- it's like having access to a detailed x-ray of the Earth. In fact, scientists discovered the jet stream thanks to bright spots in the magnetic field over Alaska and Siberia.

Swarm mission manager Rune Floberghagen said this is one of the first deep-Earth discoveries made possible by the trio of satellites. "With the unprecedented resolution now possible," he said, "it's a very exciting time -- we simply don't know what we'll discover next about our planet."