The Territory of Heard Island and McDonald Islands (abbreviated as HIMI) are an Australian external territory and volcanic group of barren Antarctic islands, about two-thirds of the way from Madagascar to Antarctica. The group's overall size is 372 square kilometres (144 sq mi) in area and it has 101.9 km (63 mi) of coastline. Discovered in the mid-19th century, they have been territories of Australia since 1947 and contain the only two active volcanoes in Australian territory. The summit of one, Mawson Peak, is higher than any mountain on the Australian mainland.