It’s a gorge about a thousand feet deep between two cirques at the bottom of which flows the Bras de Caverne River.
The canyon has two distinct parts - a large crater, which is fed by six prominent waterfalls, and a narrow slot canyon at its outlet, which constitutes most of the canyon's length. The Bras de Caverne River's headwaters are in a cirque high on the mountainside abutting the canyon wall, and directly after that, it drops over a waterfall about 700 feet (210 m) high. This drop is usually dry or has very little water but between that and the next, 600-foot (180 m) drop, springs feed the river, which drops over this then drops over a final 1,000-foot (300 m) undercut cliff into the Trou de Fer in a narrow plume of water.
Over a distance of about 3.5km the river Bras de Caverne plunges down almost 930 meter over these three dramatic waterfalls and then winds its way along the narrow canyon till it joins the Riviere du Mat on its way to the Indian Ocean.
Because of gorges like Trou de Fer created from volcanic fractures access to the center of the island of Réunion is difficult. This has protected the island’s center from human encroachment, and its tropical forests, with giant heather, ferns, and lichens, have been preserved. Forests at low altitude, however, have been converted to agricultural or urban use and have disappeared. More than 30 species of animals and plants, of which about two-thirds were endemic, have become extinct on the island in the past 400 years. The destruction of the forest and the introduction of non-native species have a serious impact on these insular ecosystems, whose balance has been created without outside influences. On the island, the dodo became extinct shortly after the arrival of Western sailors, who brought cats, rats, and pigs with them.