Here's why:

* Subduction: When two plates collide, the denser plate (usually an oceanic plate) bends and slides beneath the less dense plate (either oceanic or continental). This process is called subduction.

* Deepest Parts: The deepest parts of the ocean, the trenches, form at the point where the subducting plate bends downwards.

* Volcanoes and Earthquakes: Subduction zones are also known for their volcanic activity (like the Ring of Fire) and frequent earthquakes.

In contrast:

* Plates moving apart (diverging): This creates mid-ocean ridges, where new ocean floor is formed.

Let me know if you have more questions about plate tectonics!