The Earth's mantle is a thick layer of mostly solid rock that lies beneath the Earth's crust and above the Earth's core. Its depth varies, but generally:

* Upper Mantle: Extends from the base of the crust (about 30 km deep) to a depth of about 660 km.

* Lower Mantle: Extends from 660 km to the outer core boundary, which lies at approximately 2,890 km deep.

Therefore, the total depth of the mantle is approximately 2,900 km.

It's important to note that the mantle isn't entirely solid. It behaves like a very viscous fluid over long periods, allowing tectonic plates to move across its surface.