Sunset Crater Volcano National Monument

A thousand years ago, the ground north of Flagstaff tore open. The eruption — now dated by geologic and archaeological evidence to around AD 1085 — built a 340-meter cinder cone, sent lava flows extending kilometers in multiple directions, and buried more than 2,100 square kilometers in ash. That ash enriched the soil enough that the Sinagua people, temporarily displaced, eventually returned to farm it. In 1928, a Hollywood film company planned to detonate explosives on the crater's flank for a movie avalanche scene. Public outcry stopped them, and the national monument followed. The summit trail closed in 1973 — erosion from hikers made it necessary. The lava flow is still there, black and open, the most recent volcanic eruption in Arizona.