Rottnest Island, often colloquially referred to as "Rotto", is a 19-square-kilometre island off the coast of Western Australia, located 18 kilometres west of Fremantle. A sandy, low-lying island formed on a base of aeolianite limestone, Rottnest is an A-class reserve, the highest level of protection afforded to public land. Together with Garden Island, Rottnest Island is a remnant of Pleistocene dune ridges.