a depression in the sea bottom of the Indian Ocean at the western submarine slopes of the Amirantes. It is about 900 km long, up to 20 km wide, 4,500 m deep in the north, and 5,200 m deep in the south; its maximum depth is 5,349 m. The Amirante trough is separated from the adjacent Somali trench by a low ridge which is capped in places by mountains.