The Cat Ba archipelago forms the southwestern part of Ha Long Bay. With its 285km2, Cat Ba is the biggest island of the bay. Cat Ba is covered by karstic sceneries, but is also home to a wide variety of natural ecosystems, and a national park covering about a third of the surface of the island. Cat Ba archipelago was declared a Biosphere Reserve by UNESCO, so as to protect the multiple land and aquatic ecosystems and the diversity of plants and animals living on the island. The Cat Ba langur, for example, is endemic to the island and is one of the most endangered primates in the world.