Here's why:

* Size: The mantle makes up about 84% of Earth's total volume, extending from the crust down to the outer core.

* Thickness: The mantle is roughly 2900 kilometers (1800 miles) thick.

* Comparison: The crust is only about 30-50 kilometers (19-31 miles) thick, and the outer core is only about 2200 kilometers (1367 miles) thick.

While the Earth's atmosphere is also vast, it's not considered a "layer" in the same way as the crust, mantle, and core.