Here's why:

* Subduction Zones: Trenches form at subduction zones, where one tectonic plate (the denser one) dives beneath another. This process is called subduction.

* Oceanic-Continental Convergence: The most common type of trench formation occurs when an oceanic plate collides with a continental plate. The denser oceanic plate subducts under the continental plate.

* Oceanic-Oceanic Convergence: Trenches can also form when two oceanic plates collide. The older, denser plate subducts beneath the younger, less dense plate.

Examples:

* Mariana Trench: The deepest known point on Earth, located in the western Pacific Ocean. It was formed by the subduction of the Pacific plate beneath the Philippine plate.

* Peru-Chile Trench: Located off the coast of South America, formed by the subduction of the Nazca Plate beneath the South American Plate.

The deep trenches are a fascinating and important feature of Earth's geology, providing evidence of the constant movement and interaction of tectonic plates.