The Sundarbans region, south of Dhaka, has the largest mangrove swamp in the world (50 miles in the interior), and "the beautiful forest", an impenetrable and protected jungle of which a third of the surface is underwater. The region takes its name from the sundari tree, which can reach 82 feet high. Its very strong wood is used in naval construction, amongst other things. You will need a licence to penetrate into this region, and the presence of a guide is compulsory. You can obtain this licence from the Khulna forestry division office.