Here's why:

* Mountains form due to collisions of tectonic plates. When plates collide, one plate often subducts (slides) under the other.

* The crust is thickest under mountains. This is because the collision process pushes up the crust, creating a thick, mountainous region.

* The crust is thinnest under oceans. This is because the oceanic crust is denser and thinner than the continental crust.

Let me know if you have any other questions!