Geothermal energy is heat energy from deep in the earth. Heat is brought to the near-surface by thermal conduction and by intrusion into the earth's crust of molten magma originating from great depth. Ground water is heated to form hydrothermal resources -- naturally occurring hot water and steam. Use of hydrothermal energy is economic today at a number of high-grade sites. Hydrothermal resources are tapped by existing well-drilling and energy-conversion technology to generate electricity or to produce hot water for direct use.