Deep within our planet, intense heat left over from Earth’s formation is stoked by the continued decay of radioactive isotopes.

In a few places, like Iceland, geologic features allow this heat to come close to the surface. Here, it often turns groundwater to steam.

For a century, Iceland has been tapping into this naturally occurring steam and using it to heat the homes, streets and sidewalks of Reykjavik. Or to run power plants to make electricity.

Now, Iceland is trying to take it a step further. Scientists are drilling experimental wells, nearly 3 miles into the earth, to try to tap into superheated water.

At what’s called supercritical temperatures, above 750 degrees Fahrenheit, water and steam merge into a single supercritical fluid, which behaves differently than either.

A typical Icelandic geothermal steam well produces the equivalent of 5 megawatts of energy. A supercritical well could produce 10 times that.

This means that just three or four wells could heat an entire city.

Of course, this is a very challenging prospect. At such high temperatures and pressures, supercritical fluid is very hard to handle.

Iceland shut in its first successful well after a valve failure.

But the massive amount of energy on tap could make this a promising energy source—in those rare, lucky places where supercritical water is near enough to the surface to access.