Heard Island, barren, uninhabited subantarctic island, 142 sq mi (368 sq km), S Indian Ocean, located some 300 mi (483 km) SE of the Kerguelen Islands. Mountainous and largely covered by snow and glaciers, it is volcanic in origin. Mawson Peak, 9,006 ft (2,745 m), is an active volcano, and the highest peak in Australian territory. The island is a nature preserve and home to a large population of seals and penguins and other birds. It was discovered by U.S. Capt. John J. Heard in 1853 and and has been administered by Australia since 1947. Together with the smaller nearby McDonald Islands and Shag Island it forms the Territory of Heard Island and the McDonald Islands, 159 sq mi (412 sq km).