IN THE world of environmental activism, there is a good rule of thumb. If an energy source comes out of the ground it is probably bad (think coal, oil, natural gas and, in the view of many, uranium). If it does not, then it is probably good (think wind, waves, solar and biofuels). But there is an exception. Even the most hair-shirted environmentalist finds it hard to argue against geothermal energy. When what comes out of the ground is merely hot water or steam there is, as it were, little to get steamed up about.

The problem is that traditional geothermal power relies on volcanism. Fine if you live in Iceland or New Zealand. Not so good in a geologically passive place such as Germany. Which is why Wulf Brandt, of the National Research Centre of Geosciences in Potsdam, has dug a deep hole in the ground at Gross Schönebeck, near Berlin.

Dig deep enough and geothermal energy is everywhere. That is because the temperature increases by 25-30°C for every kilometre you go down. Useful electricity generation needs a temperature above 150°C. In the case of the rocks under Gross Schönebeck, that means digging to a depth of 4.4km. Also, to extract the heat, you need two holes. Unlike volcanic geothermal energy, supplied by water or steam gushing to the surface unprompted, the non-volcanic variety requires a pump—and two holes in the ground rather than one. Cold water goes down the first and hot water comes up the second.

One reason Mr Brandt picked Gross Schönebeck was that one of these two holes was available for nothing. It had been drilled by a gas exploration company, but had proved “dry”. Mr Brandt drilled the other so that it was the same depth as the first, but it bottomed out 400 metres way. That, he reckoned, was far enough for water travelling between the two holes to heat up without cooling the intervening rocks too rapidly.

The question was, would water pushed down the first hole force water up the second in large enough quantities and at high enough temperatures to be commercially viable? To start with, the answer was no. But as the experiment progressed, the rate of flow increased—and this was no accident.

One of the things Mr Brandt had taken into account when he planned the experiment was the grand movement of the Earth's tectonic plates. He aligned the two holes in a way that would benefit from the tension created as far north as Berlin by the collision of African and Eurasian plates that uplifted the Alps. The downward-flowing water, which was injected at a pressure of 500 atmospheres, gradually forced apart existing fissures, and created new ones, in such a direction that the tension helped keep them open.

Another trick he employed was to add grains of sand and corundum to the water. These grains were the right size to help keep the fissures open without blocking them. The result was that the rate of flow increased towards 75 cubic metres an hour, which is needed to make an electricity plant operable.

It remains to be seen whether that flow rate can be sustained, and Gross Schönebeck thus turned into a commercial power station. But the auspices are good. If it can, it will pave the way for others. The supply of heat from the Earth's interior is, in practice, inexhaustible—and although any given site can be cooled to the point where it is useless (Dr Brandt reckons that would take about 30 years in the case of Gross Schönebeck), you can always dig more holes.