In a revival of the French capital's geothermal potential, drilling has just begun in the north of the city on a desolate building site sandwiched between the traffic-clogged inner ring road and the Saint-Denis canal.

"In Paris we're trying to adopt a strategy in which France is largely behind other European countries, because we've under-invested in renewable energies," said Denis Baupin, a Paris deputy mayor.

At the construction site, a 36-metre (120-foot) yellow mast rises above a dense cluster of machinery that is usually used to drill for oil. Here the drilling is not for black gold but for hot water.

"The lower you go, the hotter the water," explained Michel Galas of CPCU, the urban heating company carrying out the work, as he stood next to a shaft that when finished will delve 1.7 kilometres (one mile) into the earth.

At that depth lies a geological stratum called the Dogger from which water, heated naturally to 57 degrees Celsius (135 Fahrenheit), will be sucked up to the surface, where it will be used to heat another stock of water.

This will be pumped to apartment blocks to heat radiators and provide hot water.

"It's energy that is 100 percent renewable," said Galas, adding that drilling will go on night and day for about 100 days to reach the required depth.

The scheme will heat around 12,000 apartments and other buildings due to be built by 2011 in a new residential area in the city's 19th district. The project will cost 31 million euros (40 million dollars), five million euros of which will come from the state environment agency and the regional council.

Galas, whose company is jointly owned by the City of Paris and the energy group GDF Suez, pointed to a row of high-rise tower blocks on the other side of the ring road and said they too would eventually be hooked up to the system.

The use of this natural energy source will prevent 14,000 tonnes a year of the main greenhouse gas carbon dioxide being pumped into the capital's already polluted air.

That is roughly the same amount of CO2 that an average car would belch out if taken on a 470,000-kilometre (290,000-mile) trip, which is longer than the distance from the Earth to the moon.

It will also provide 54 percent of the new area's energy needs.

Galas said there were around three dozen sites using geothermal energy in the greater Paris region, nearly all dating from the 1970s and 80s.

"For about 25 years there were no new projects because the price of a barrel of oil had gone down, but in recent years there has been a growing awareness of environmental issues, combined with a hike in the price of oil," he said.

With the environment placed high on President Nicolas Sarkozy's political agenda, and with Baupin and his Green party as the Socialists' coalition partners in Paris city hall, the time appeared right to return to geothermal.

-- Geothermal energy in use since Roman times --

Geothermal energy has been used since Roman times to heat buildings. It uses the energy recovered from the heat of the earth's core and can be seen naturally in the form of volcanoes, geysers and hot springs.

Heated water drawn to the surface can, if of sufficiently high temperature, also be used to drive turbines to create electricity.

In Iceland, about a quarter of the island nation's electricity comes from geothermal power plants, while geothermal schemes provide heating and hot water for almost nine tenths of the country's buildings.

France's geology does not permit tapping into geothermal energy on the Icelandic scale, but there is much unused potential.

Currently around 170,000 French homes are heated geothermally, but the government plans to multiply that number by six by 2020, which would mean that four percent of the nation's households would be thus heated.

The Paris area, Alsace in the east, and Aquitaine in the southwest are the regions geologically best suited for such projects.

Baupin, the deputy Paris mayor in charge of sustainable development and the driving force behind the city's wildly successful Velib bike-sharing scheme, said a second project is already being developed in the north of the city.

And the capital's second airport, Orly, a year ago announced plans to extract geothermal energy to slash its heating bills.

Galas of the CPCU could not provide figures comparing geothermal energy costs with costs from using gas or oil alternatives, but he said geothermal heating was financially "competitive" compared to gas.

Baupin promised that residents of the new area in north Paris "will certainly be a lot less exposed to the vagaries of the price of oil or gas."

Experts agree.

At talks on climate change in December in Poland, UN Environment Programme Executive Director Achim Steiner spoke of geothermal as clean and "indigenous" -- code for free from geopolitical risk and immune to market fluctuations.

"We're not trying to make Paris self-sufficient in energy, that would be too ambitious," said Baupin, who nevertheless talks enthusiastically of other green energy projects such as harnessing the flow of the River Seine.

"But we should aim to make ourselves less vulnerable to future energy crises," he said.

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