Rùm is mainly igneous in origin, and its mountains have been eroded by Pleistocene glaciation. It is now an important study site for research in ecology, especially of red deer, and is the site of a successful reintroduction programme for the white-tailed sea eagle. Its economy is entirely dependent on NatureScot, a public body that now manages the island, and there have been calls for a greater diversity of housing provision. A Caledonian MacBrayne ferry links the island with the mainland town of Mallaig.