Geothermal energy is a very powerful and efficient way to extract a renewable energy from the earth through natural processes. This can be performed on a small scale to provide heat for a residential unit (a geothermal heat pump), or on a very large scale for energy production through a geothermal power plant. It has been used for space heating and bathing since ancient roman times, but is now better known for generating electricity. Geothermal power is cost effective, reliable, and environmentally friendly, but has previously been geographically limited to areas near tectonic plate boundaries. Recent technological advances have dramatically expanded the range and size of viable resources, especially for direct applications such as home heating. The largest group of geothermal power plants in the world is located at The Geysers, a geothermal field in California, United States. As of 2004, five countries (El Salvador, Kenya, the Philippines, Iceland, and Costa Rica) generate more than 15% of their electricity from geothermal sources. Geothermal power requires no fuel, and is therefore immune to fluctuations in fuel cost, but capital costs tend to be high. Drilling accounts for most of the costs of electrical plants, and exploration of deep resources entails very high financial risks. Geothermal power offers a degree of scalability: a large geothermal plant can power entire cities while smaller power plants can supply rural villages or heat individual homes. Geothermal electricity is generated in 24 countries around the world and a number of potential sites are being developed or evaluated.