The temperatures in the earth’s interior are very high. 99% of the planet is hotter than 1,000° C, and only 0.1% is cooler than 100° C. This vast geothermal energy reservoir can be exploited with the aid of suitable methods, I read here. Geothermal power is energy generated by heat stored in the earth, or the collection of absorbed heat derived from underground, in the atmosphere and oceans. As of 2008, geothermal power supplies less than 1% of the world's energy, according to Wikipedia.
Geothermal power belongs to the energy sources that Stadt Zürich wants to use to become a 2000 Watt society. Currently, each person in Switzerland needs, on average, 6000 watts per person per year.
Starting, according to this recent article, in Spring 2009, Elektrizitätswerk der Stadt Zürich (EWZ) will drill down to 3.2 kilometres in order to possibly generate energy to use for heating and electricity. For every 100 metres you drill down in the earth, the temperature increases by approximately 3 degrees celsius, I read in another article about the topic.