That area south of ancient Egypt proper which extends from the Nile Valley from Aswan and the first cataract as far south as the Khartoum district in the Sudan, east to the Red Sea and west to the Libyan Desert. It was conventionally divided into Upper and Lower Nubia. Most of the Egyptian section was submerged under Lake Nasser since the Aswan High Dam completion in 1971. Defined 'corridor to Africa', Nubia was a crucial trading conduit from 4th millennium BC until Middle Ages. The southern part of it to the southern end of the second cataract of the Nile was called Cush (Kush) under the 18th-dynasty pharaohs of ancient Egypt and called Ethiopia by the ancient Greeks. The northern part of the region, up to the first cataract of Aswan, was called Wawat.