Geothermal energy is the natural heat of the Earth.

2. Geothermal energy is the natural heat of the Earth.

3. Heat flows outward from Earth's interior. The crust insulates us from Earth's interior heat. The mantle is semi-molten, the outer core is liquid and the inner core is solid.

4. The deeper you go, the hotter it gets (in Fahrenheit and miles).

5. The deeper you go, the hotter it gets (in Fahrenheit and miles).

6. Earth's crust is broken into huge plates that move apart or push together at about the rate our fingernails grow. Convection of semi-molten rock in the upper mantle helps drive plate tectonics.

7. New crust forms along mid-ocean spreading centers and continental rift zones. When plates meet, one can slide beneath another. Plumes of magma rise from the edges of sinking plates.

8. Thinned or fractured crust allows magma to rise to the surface as lava. Most magma doesn't reach the surface but heats large regions of underground rock.

9. Rainwater can seep down faults and fractured rocks for miles. After being heated, it can return to the surface as steam or hot water.

10. This steaming ground is in the Philippines.

11. When hot water and steam reach the surface, they can form fumaroles, hot springs, mud pots and other interesting phenomena.

12. When the rising hot water and steam is trapped in permeable and porous rocks under a layer of impermeable rock, it can form a geothermal reservoir.

13. A geothermal reservoir is a powerful source of energy!

15. Many areas have accessible geothermal resources, especially countries along the circum-Pacific "Ring of Fire," spreading centers, continental rift zones and other hot spots.

16. These and other methods are used.

17. Exploration commonly begins with analysis of satellite images and aerial photographs.

18. Volcanoes are obvious indications of underground heat, this volcano, Mt. Mayon in the Albay province of the Philippines erupted in 1999.

19. Geologists explore volcanic regions to find the most likely areas for further study, like this steaming hillside in El Hoyo, Nicaragua.

20. Geologic landforms and fault structures are mapped in the region. This view overlooks Basin and Range terrain East of the Sierra Nevadas.

21. Rocks are examined up close.

22. Geologic maps like this one are created, showing rock type and ages in different colors.

23. Data from electrical, magnetic, chemical and seismic surveys is gathered in the field.

24. The data obtained in the field are displayed in various ways and analyzed.

25. Geologists and drillers study the data to decide whether to recommend drilling. Geothermal reservoirs suitable for commercial use can only be discovered by drilling.

26. First, a small- diameter "temperature gradient hole" is drilled (some only 200' deep, some over 4000 feet deep) with a truck-mounted rig to determine the temperatures and underground rock types.

27. Workers on a temperature gradient hole drilling project.

28. Either rock fragments or long cores of rock are brought up from deep down the hole and temperatures are measured at depth.

29. Geologists examine the cored rock (shown here marked with depth markers).

30. Temperature results like this would definitely encourage the drilling of a larger, deeper well to try to find a hydrothermal reservoir.

31. Production-sized wells require large drill rigs like these and can cost as much as a million dollars or more to drill. Geothermal wells can be drilled over two miles deep.

32. On these large rigs, drilling continues 24 hours per day.

33. If a reservoir is discovered, characteristics of the well and the reservoir are tested by flowing the well.

34. If the well is good enough, a wellhead, with valves and control equipment, is built onto the top of the well casing.

35. This photograph shows a vertical geothermal well test in the Nevada Desert.

37. Natural steam from the production wells power the turbine generator. The steam is condensed by evaporation in the cooling tower and pumped down an injection well to sustain production.

38. Like all steam turbine generators, the force of steam is used to spin the trubine blades which spin the generator, prducing electricity. But with geothermal energy, no fuels are burned.

39. Turbine blades inside a geothermal turbine generator.

40. Turbine generator outdoors at an Imperial Valley geothermal power plant in California.

41. Turbine generator in a geothermal power plant in Cerro Prieto, Mexico.

42. Geothermal power plant operators in geothermal power plant control room in the Philippines.

43. Substation with transformer and insulators, at a geothermal power plant.

44. Wood power poles delivering electricity from geothermal power plants in the Mojave Desert in California to the electrical grid. Steam from well-testing in background.

45. Those white plumes you see at geothermal power plants are steam (water vapor). Geothermal plants do not burn fuel or produce smoke.

46. Geothermal power plants are clean and are operating successfully in sensitive environments.

47. These geothermal plants are operating successfully in a Philippine cornfield, at Mammoth Lakes, Calif., in the Mojave Desert of California, and in a tropical forest, at Mt. Apo, Philippines.

48. There are different kinds of geothermal reservoirs and different kinds of power plants.

49. In dry steam power plants, the steam (and no water) shoots up the wells and is passed through a rock catcher (not shown) and then directly into the turbine. Dry steam fields are rare.

50. Prince Piero Ginori Conti invented the first geothermal power plant in 1904, at the Larderello dry steam field in Italy.