By Martin Redfern
BBC Radio science unit
Geologists in Iceland are drilling directly into the heart of a hot volcano.
Land of steam: Iceland sits atop the Atlantic's mid-ocean ridge system
Their $20m project will lead to boreholes that could ultimately yield 10 times as much geothermal power as any previous project.
It is hoped the endeavour will also reveal more about the nature of mid-ocean ridges where new ocean floor is created.
Twenty years ago, geologist Gudmundur Omar Friedleifsson had a surprise when he lowered a thermometer down a borehole.
"We melted the thermometer," he recalls. "It was set for 380C; but it just melted. The temperature could have been 400 or even 500."
Speaking in the first of a new series on BBC Radio 4, called Five Holes in the Ground, he describes how this set him thinking about how much energy it might be possible to extract from Iceland's volcanic rocks.
At depth, the groundwater is way over 100C, but the pressure keeps it liquid. As Dr Friedleifsson puts it: "On the surface, you boil your egg at 100 degrees; but if you wanted to boil your egg at a depth of 2,500m, it would take 350."
The landscape on the Reykjanes Ridge in southwest Iceland seems like an alien world.
There are pools of boiling mud and the hiss of steam escaping from fissures. There are also signs of industry - past, present and future - with an abandoned salt factory, working geothermal power stations and a big new drilling rig.
It is also an area of great natural beauty. Down on the shore, crashing Atlantic breakers are exposing fresh cliffs of pillow basalt, volcanic lava that has erupted under the sea and been rapidly quenched so that it forms features that look a bit like black toothpaste squeezed from a giant tube.
This is a young landscape. The most recent eruptions here occurred in the 13th Century and there could be new ones at any time.
Iceland is unusual geologically in that it exists above the ocean at all. It stands on the mid-ocean ridge system, the longest mountain range on the planet. This range runs around the world's oceans like the seam on a tennis ball.
It is here that new ocean floor is created as the continents drift apart.
For the most part, it is deep under the sea; it is the place where hydrothermal vents and their "black smokers" belch out super-heated water and dissolved minerals.
But Iceland stands on an additional plume of volcanic mantle rock that has lifted it above the Atlantic and made it accessible to geologists.
Some 90% of all homes in Iceland are heated by geothermal energy; and a number of power stations are also producing electricity from steam at around 240C, extracted from boreholes between 600 and 1,000m deep.
But now, the plan is to go much deeper. Omar Friedleifsson of the Iceland Geosurvey is leading the consortium of energy companies in the Iceland Deep Drilling Project.
Last year, they drilled down to a depth of 3,082m and since then have been conducting flow tests.
Later this year, they will put a pressure lining into their borehole and drill on down to more than 4km deep.
At that depth, they hope to encounter what is called supercritical water: water that is not simply a mixture of steam and hot water but a single phase which can carry much more energy.
Engineers on the project have calculated that increasing the temperature by 200 degrees and the pressure by 200 Bar will mean that, for the same flow rate, the energy extracted from such a borehole will go up from 5MW to 50MW.
Power station manager Albert Albertsson predicts that, by the end of the century, "Iceland could become the Kuwait of the North". The vision is to use this cheap and carbon-free energy to split water, to yield hydrogen that could be despatched around the world in tankers.
But interest in the Iceland Deep Drilling Project is not solely for energy production.
Geologists have never had the chance before to penetrate the volcanic heart of a mid-ocean-ridge geothermal system and there is much they would like to learn.
As they get deeper, bore teams will change from the rotary drill, which produces rock fragments but can drill up to 200m per day, to a slower drill that produces useful core samples.
The project wants to study the geology, the energy flow and the chemical environment at great depth.
Albert Albertson, at the nearby power station, likes to think of the energy as just a part of an integrated system.
Blue Lagoon: A perfect place for a dip, especially during winter
Iceland's volcanic rocks are highly fractured and so, below about 50m, there is plenty of water.
For the next 40m or thereabouts, it is fresh drinking water, topped up by Iceland's generous rainfall. Below that, the water is salty; the ocean has managed to seep in.
However, it is the really deep supercritical water that is also laden with dissolved minerals. Mr Albertsson believes he may also be able to extract precious metals, such as copper, silver and gold from the water.
After the water has gone through his turbines, it is still at about 40C. Some of that excess energy is used for district heating and for horticulture in greenhouses.
It also warms one of Iceland's biggest tourist attractions: the Blue Lagoon, a vast outdoor lake which, even in March, greets bathers with the temperature of a hot bath.
There are supposed benefits from the silica rich water with its faint smell of sulphur, and the white silica mud is exported for health and beauty treatments.
Mr Albertsson told the BBC that he himself is a regular visitor.
"For me, the ideal time to take a dip is in the middle of winter, in the middle of the night, looking up at the stars and the Aurora Borealis, the Northern Lights."
Five Holes in the Ground is a series of five programmes broadcast on BBC Radio 4 in the week 27-31 March. You can listen to the programmes after they are broadcast on the website.