An underwater canyon where oceanic crust bends downward is called a subduction zone.

Here's why:

* Subduction: This is the process where one tectonic plate (the denser one) slides beneath another.

* Oceanic Crust: The crust that makes up the ocean floor is denser than continental crust.

* Bending Downward: As the denser oceanic plate moves towards a continental plate, it is forced to bend downwards at the subduction zone. This bending creates a deep trench, a characteristic feature of subduction zones.

Let me know if you have any other questions about plate tectonics!