The river lies 3,000 kilometres beneath the surface of Earth. It's 420 km wide and 7,000 km long and probably stretches around 5,000 km downwards. It's also nearly as hot as the surface as the sun, according to the results published in the journal Nature Geoscience by scientists from the Technical University of Denmark (DTU), who used data from the European Swarm satellites to find the river.
Satellites like these detect changes in the Earth's magnetic field, which is controlled by the liquid iron and nickel churning around in the Earth's core. By measuring fluctuations in the magnetic field to very high accuracy, the team was able to detect the molten river, which they refer to as the "jet-stream."
They compare it to atmospheric jet streams, which are fast flowing, narrow air currents that sweep across the Earth at high altitudes in a meandering shape. The molten river works similarly but under the ground.
The molten jet stream has sped up recently, according to the researchers. It's speed has tripled in the last 15 years. Exactly what drives the motion and these changes is unknown so far, but the team thinks that it's probably triggered by changes in the magnetic field within the core.
The river moves at a speed of 45 km per year at the moment
That's 5 meters per hour.
"Forty-five kilometres a year may not sound like much," Chris Finlay, Swarm project scientist and senior scientist at DTU Space at Denmark's Technical University in Copenhagen, said in a statement, "but it is the fastest we have seen something move in the Earth's interior, and three times faster than anything else in the Earth core."
He said that as well as generating the Earth's magnetic field, the jet-stream may also be causing changes in the rotation rate of the Earth's inner core.
"By better understanding the physics of the core we should be able to eventually better forecast future changes in the Earth magnetic field," he added.
Our planet's magnetic field shields us from damaging radiation from space and it also has an impact on navigation systems, and many birds rely on it to find their way. So it's important to monitor the constant changes it goes through.
Scientists measure the Earth's magnetic field with the European Swarm satellites, which were launched into space in 2013.
There are three in total. Two of them are called Alpha and Charlie, and they fly in tandem at about 450 km away from Earth. The third is called Bravo, and it's in orbit 500 km from Earth.
The way they move around each other in orbit allows effects of other magnetic sources - or those not coming from Earth - to be ignored. This means the signal from the core is uninterrupted and clear. By monitoring the field closely, scientists hope to better understand the processes that affect climate and weather.