Deep-sea trenches are long, v-shaped, steep-sided valleys on the ocean floor. Their formation is due to plate tectonic activity. When the leading edge of an oceanic plate meets the leading edge of a continental plate, the crust interacts. Oceanic crust is denser than continental crust, causing the oceanic crust to bend downward. This movement creates a deep v-shape, which becomes a trench. Deep-sea trenches are found between 24,000 to 36,000 feet under water. The deepest known trench is the Mariana Trench in the western portion of the North Pacific Ocean. Its depth is 36,200 feet.