A geothermal reservoir is a natural underground area where heat from the Earth's interior is stored. This heat can come from various sources, including the decay of radioactive materials and the residual heat from the planet's formation. These reservoirs typically contain hot water or steam, which can be accessed for energy production or direct heating applications. Geothermal reservoirs are often found in regions with volcanic activity, such as near hot springs or geysers. They are an important resource for renewable energy, as they can provide a consistent and sustainable source of power. By tapping into these reservoirs, we can harness the Earth's heat for electricity generation and heating purposes.