The Earth's crust is thickest under continental shields, specifically under the oldest and most stable parts of continents, often called cratons. These regions are typically found in the interior of continents and have been relatively undisturbed by tectonic activity for billions of years.

Here's why:

* Continental Crust: Continental crust is thicker and less dense than oceanic crust. It's made up of a variety of igneous, metamorphic, and sedimentary rocks.

* Shield Areas: These areas have experienced minimal erosion and have accumulated layers of rocks over time, leading to a thicker crust.

* Stable Regions: Since they are relatively inactive tectonically, these areas haven't been subjected to the forces that can thin or break apart the crust (like mountain building or subduction).

Examples of areas with thick crust:

* Canadian Shield: Located in North America.

* Baltic Shield: Located in northern Europe.

* Australian Shield: Located in Australia.

It's important to note that even within these shield areas, the crust thickness can vary, but it's generally thicker than other parts of the Earth's crust.