As its name suggests, this depression comes in the form of an abyss of almost 300 meters profondeur north of the plateau formed by the forest Bélouve, west of the home forest of the Plaine des lianas and is a rampart overhung by the summit called the Spur that dominates also the place called Salazie. The many irregular water courses whose Cavern arm and Mazerin ravine that rush Hole in the Iron form a drop that reaches 225 m in altitude, the highest waterfall in the French territory and one of the highest waterfalls in the world. Below, the Cave arm escapes from the north through a narrow gorge and empties into the River Mast, river escaping meanwhile the Salazie.