The Fort du Trou-d'Enfer, also known as the Réduit du Trou-d'Enfer, is one of the fortifications of Paris, It is located in Marly-le-Roi, in the departement of Yvelines. The fort was built between 1878 and 1881 for a garrison of 800 men. It was termed a réduit for its position surrounded by several smaller batteries.