Geothermal energy comes from the heat stored within the Earth. This heat originates primarily from two sources: the original formation of the planet and the ongoing radioactive decay of particles deep inside the Earth's core

. The Earth's internal structure consists of:

• A very hot inner core of solid iron (about 1,500 miles in diameter)
• An outer core of molten rock called magma (about 1,500 miles thick)
• A mantle of magma and rock surrounding the core (about 1,800 miles thick)
• A solid crust forming the continents and ocean floors (15 to 35 miles thick under continents, 3 to 5 miles thick under oceans)

Heat from the mantle rises through the Earth's crust, especially where the crust is thin or fractured due to tectonic plate activity

. This heat warms underground reservoirs of water trapped in porous rocks or fractures, creating geothermal reservoirs of hot water and steam

. These reservoirs are often found near tectonic plate boundaries, where volcanic activity brings magma closer to the surface, such as along the Pacific "Ring of Fire"

. Geothermal energy naturally reaches the surface through features like volcanoes, hot springs, geysers, and fumaroles (holes releasing volcanic gases)

. Humans access this energy by drilling deep wells into these hot underground reservoirs to extract steam or hot water, which can then be used for electricity generation, heating, and other applications

. In summary, geothermal energy comes from the Earth's internal heat generated by radioactive decay and residual planetary formation heat, accessed mainly where heat is close to the surface due to geological activity