An ocean basin is one of two major provinces of the deep ocean floor, lying over 2 kilometers (1.2 miles) in depth. The mid-ocean ridges form the other province. Together they constitute 50% of the Earth's surface. The deep ocean basin is underlain by a basaltic crust, about 7 kilometers (4.3 miles) thick, and is covered in sediment and dotted by low abyssal hills. See also ocean trench.