The part of the Earth that is approximately 2900 km thick is the mantle.

Here's a breakdown of the Earth's layers and their thicknesses:

* Crust: 5-70 km thick (varies depending on whether it's oceanic or continental)

* Mantle: 2900 km thick

* Outer Core: 2200 km thick (liquid)

* Inner Core: 1220 km thick (solid)