Sentinel Peak, known locally as “A” Mountain, rises above Tucson as both a natural landmark and a place layered with Indigenous, agricultural, military, civic, and geologic history. The city’s name derives from the O'odham Cuk Ṣon, “the base of the mountain is black,” and the land below supported farming along the Santa Cruz River from about 2000 BCE into the 1930s. After Presidio San Agustín del Tucsón was built in 1775, guards watched from the summit, leaving fortification remains. In 1916, University of Arizona students built the giant hillside “A,” which became a lasting symbol of Tucson. The peak’s igneous rocks also preserve evidence of ancient volcanic activity.