The South Forty-Foot Drain, also known as the Black Sluice Navigation, is the main channel for the land-drainage of the Black Sluice Level in the Lincolnshire Fens. It lies in eastern England between Guthram Gowt and the Black Sluice pumping station on The Haven, at Boston. The Drain has its origins in the 1630s, when the first scheme to make the Fen land available for agriculture was carried out by the Earl of Lindsey, and has been steadily improved since then.