Organ Pipe Cactus National Monument protects a stretch of the Sonoran Desert in southern Arizona, near the U.S.–Mexico border. It is best known for being the only place in the United States where the multi-stemmed organ pipe cactus grows naturally in abundance, along with many other desert plants. Organ Pipe Cactus National Monument is 517 sq mi (1,340 km2) in size. In 1976 the monument was declared a Biosphere Reserve by UNESCO, and in 1977 95% of the monument was declared a wilderness area.