Saguaro National Park preserves 92,000 acres of Sonoran Desert in two districts on either side of Tucson, protecting giant saguaro cactus habitat and two contrasting mountain landscapes. The lower volcanic Tucson Mountains in the west differ sharply from the higher, wetter Rincon Mountains in the east, part of the Madrean Sky Islands, reflecting major geologic change over millions of years. Human presence reaches back to the Hohokam, with later Native peoples, Spanish explorers, and settlers also using the land. Federal protection began in 1933, expanded in 1961, and the two districts were united as a national park in 1994, with much of the area now wilderness.