Mumbai Harbour (Marathi: मुंबई बंदर Mumba'ī bandar), or Front Bay (sometimes referred as Mumbai Harbour), is a natural deep-water harbour in the southern portion of the Ulhas River estuary. The narrower, northern part of the estuary is called Thane Creek. The harbour opens to the Arabian Sea to the south. The historical island of Elephanta is one of the six islands that lie in the harbour. The Harbour Line of the Mumbai Suburban Railway system runs along the western edge of the harbour.