Trenches are the deepest parts of the ocean. Many renches occur in the Pacific Ocean, especialy its in wesstern portion. Most trenches are long, narrow, and deep, 2 to 2.5 miles (3 to 4 km) below the the surrounding sea floor. The greaest depth anywhere in the ocean is found in the Mariana Trench southeast of Japan. It pluges more than 6.8 miles (11km) below sea level. Frequent earthquakes and volcanic eruptions occur along the trenches.