Here's why:

* Depth and Pressure: The mantle lies beneath the crust, extending from about 30 kilometers (19 miles) to 2,900 kilometers (1,800 miles) deep. The immense pressure at these depths increases the temperature significantly.

* Heat Sources: The mantle's heat comes primarily from the Earth's core, which is extremely hot due to radioactive decay. This heat is transferred upwards through the mantle, gradually decreasing as it approaches the surface.

* Temperature Gradient: The Earth's interior has a temperature gradient, meaning temperatures increase with depth. The crust's average temperature is about 20°C (68°F), while the mantle's temperature can reach up to 4,000°C (7,232°F) near the core-mantle boundary.

While the crust is the outermost layer, it's relatively cool compared to the incredibly hot temperatures found deep within the Earth.