LONG WAY DOWN: Iceland drills 4.7 km down into volcano to tap clean energy.

At this depth, engineers hope to access hot liquids under extreme pressure and at temperatures of 427 degrees C (800 F), creating steam that turns a turbine to generate clean electricity.

Iceland’s decision to harness the heat inside the earth in a process known as geothermal energy dates back to the 1970s and the oil crisis.

But the new geothermal well is expected to generate far more energy, as the extreme heat and pressure at that depth makes the water take the form of a “supercritical” fluid, which is neither gas nor liquid.

“We expect to get five to 10 times more power from the well than a conventional well today,” said Albert Albertsson, an engineer at the Icelandic energy company HS Orka, involved in the drilling project.

To supply electricity and hot water to a city like Reykjavik with 212,000 inhabitants, “we would need 30-35 conventional high temperature wells” compared to only three or five supercritical wells, says Albertsson. The cost would be much less.

On a remote island nation with lots of volcanic activity, geothermal would seem to make sense. But even after digging “Thor,” scientists “have two years to determine its success and the economic feasibility of the experiment.”