Sundarbans, formerly Sunderbunds, vast tract of forest and saltwater swamp forming the lower part of the Padma (Ganges [Ganga])-Brahmaputra River delta in southeastern West Bengal state, northeastern India, and southern Bangladesh. The tract extends approximately 160 miles (260 km) west-east along the Bay of Bengal from the Hugli River estuary in India to the western segment of the Meghna River estuary in Bangladesh and reaches inland for about 50 miles (80 km) at its broadest point. A network of estuaries, tidal rivers, and creeks intersected by numerous channels, it encloses flat, densely forested, marshy islands. The total area of the Sundarbans, including both land and water, is roughly 3,860 square miles (10,000 square km), about three-fifths of which is in Bangladesh.