Ocean trenches are long, narrow valleys that form the deepest parts of the ocean plunging 3-4km (2-2.5 miles) below the adjacent sea floor. The deepest is the Marianas Trench south east of Japan near the Philippines, which reaches depths of nearly 36,000ft (11km) below sea level. Deep sea trenches are formed either by collision between two tectonic plates, or when the moving sea floor collides with continental land masses and is forced sharply downwards, often creating a trench on the seaward side and a mountain range on the landward side, such as the Andes in South America and the Peru-Chile Trench.