The thickest crust on Earth is found under continental shields, which are the oldest and most stable parts of the continents. These areas, like the Canadian Shield or the Baltic Shield, typically have crust that is 35 to 40 kilometers thick.

Here's why:

* Older crust is thicker: Continental shields have been around for billions of years, allowing them to accumulate more layers of rock over time.

* Less tectonic activity: These areas are relatively inactive compared to other parts of the Earth's surface, meaning they haven't been subjected to the same level of erosion or deformation.

* Isostatic equilibrium: The weight of the thick crust causes it to sink deeper into the mantle, maintaining an equilibrium.

While oceanic crust is much thinner (typically 5 to 10 km), it's important to note that the Earth's crust is not uniform. It's constantly changing, and the thickness can vary within a region.