In Jules Verne’s science fiction novel, Journey to the Center of the Earth, an intrepid group of explorers descends deep into a system of caves purported to lead to the center of the earth. To their surprise, they emerge on the shores of a vast underground sea complete with prehistoric creatures and exotic plants. After floating around the sea on a homemade raft, they end up being blasted back to the surface in a volcanic eruption.

It might be a bit different then Verne imagined it, but scientists have recently discovered evidence that vast stores of water might be present 500 km beneath Earth’s surface. Instead of existing as a vast underground ocean, this water is thought to be locked up in the structure of a mineral called ringwoodite.

The evidence is in the form of a tiny, battered diamond found in a shallow river bed in Brazil. The diamond, which is only five millimeters long, is scarred and ugly to look at, is probably worth only about $20. But to the geologists who got their hands on it, it’s priceless.

The low nitrogen content of the diamond provides evidence that it formed in the so called “transition zone”, at a depth of around 500 km.

Like the explorers in Journey to the Center of the Earth, the diamond was blasted to the surface in a volcanic eruption after a several-days journey from the 500 km depth where it formed.

The diamond has given scientists a rare chance to study the composition of the earth at this great depth – traditional drilling methods, of course, can only sample the upper tens of kilometers of crust (see this article on a deep drilling attempts). and scientists usually have to rely on indirect methods to look deeper.

What surprised scientists most was the presence of ringwoodite within the diamond. Ringwoodite is a form of olivine that forms at extremely high temperatures and pressures. Although its presence in the transition zone has long been theorized, this is the first time any physical evidence has been found.

The biggest surprise was the water content of the ringwoodite – 1.5%. Given the huge amount of ringwoodite expected to be present underground, this modest percentage could represent a volume of water greater than all the water in the oceans – albeit trapped within the chemical structures of solid crystals.

The presence of water underground would help explain processes in plate tectonics that have been unexplained so far – water interacting with molten rock causes changes in its physical properties.

full report of the discovery can be found in the journal Nature.