The Monument is home to twenty-six species of cacti, including its namesake, the organ pipe cactus. More than seventy-five miles of accessible vehicle roads wind through two, distinct habitat zones of the Sonoran Desert – the hotter and drier lower Colorado zone and the more luxuriant Arizona upland, depicted in this exhibit. The Monument is located in southern-most central Arizona. To its east lies the Tohono O’Odham Indian Reservation. To the west is Cabeza Prieta National Wildlife Refuge – 860,000 acres of wild, rugged Sonoran Desert, accessible only by four-wheel-drive vehicle and special permit. In 1976, the United Nations designated Organ Pipe Cactus National Monument an international Biosphere Reserve for its unique family of wildlife and plants.