Oceanic trench
Ocean trenches are typically caused when an oceanic crustal plate slides under a lighter continental plate or another oceanic plate. It is believed to work by density.  Where one plate slides under another, this is called subduction. As the heavier plate descends, the long, narrow feature caused is called the "subduction zone." It is here that oceanic trenches form. They can be 1,500 miles (2,400 km) long, several miles deep, and as much as 70 miles (112 km) wide.