The part of the Earth that is about 3500 km thick is the mantle.

Here's a breakdown of Earth's layers:

* Crust: The outermost layer, thin and rigid, about 5-70 km thick.

* Mantle: The thickest layer, extending from the crust to the outer core. It's about 2900 km thick, making up about 84% of Earth's volume.

* Outer core: A liquid layer of iron and nickel, about 2200 km thick.

* Inner core: A solid sphere of iron and nickel, about 1220 km thick.

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