Here's why:

* Heat Source: The Earth's core is incredibly hot, with temperatures reaching thousands of degrees Celsius. This heat is generated from the decay of radioactive elements within the Earth.

* Mantle Convection: The mantle is a layer of hot, semi-solid rock that constantly moves due to convection currents. These currents are driven by the heat from the core.

* Thermal Energy Transfer: As the hot mantle material rises, it transfers heat to the Earth's crust. This heat can manifest in various ways, including:

* Geysers and Hot Springs: Where water interacts with hot rocks near the surface.

* Volcanic Activity: Where molten rock (magma) from the mantle reaches the surface.

* Geothermal Reservoirs: Underground areas where hot water or steam is trapped, providing a source of energy.

So, while the Earth's core is the ultimate source of heat, geothermal energy is ultimately extracted from the Earth's mantle through the processes described above.