The layer of the Earth that is approximately 3500 km thick is the mantle.

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

* Crust: 5-70 km thick (varies greatly)

* Mantle: 2900 km thick (upper mantle: 660 km, lower mantle: 2240 km)

* Outer Core: 2200 km thick

* Inner Core: 1220 km thick