Island Park covers the northern part of Fremont County from Ashton Hill to the Montana border and contains an ancient volcanic caldera that has been inactive for the last 600,000 years, one of the world's largest at about 350 square miles. From 9,866 foot Mt. Sawtell, reached by a mountain road, the caldera can be seen, and the radar dome there is part of North America's air traffic control system. Island Park is home to the headwaters of Henry's Fork, a blue ribbon trout stream fed by Henry's Lake and Big Springs, where 120 million gallons of pure water flow daily and the Big Springs National Waterway begins a short distance downstream. Andrew Henry passed through the area in 1810 with fur trappers, including some who had been with Lewis and Clark's Corps of Discovery, and Henry's Fork bears his name. Island Park Reservoir, created by a dam built across Henry's Fork in 1938, extends 11 miles and is popular for trout fishing and boating. Nearby historical and natural sites include the Fort Henry monument associated with the Henry Expedition of 1810-1811; the Fremont County Sugar Company slicer, which operated from 1904 to 1914 processing sugar beets; the shifting St. Anthony Sand Dunes; the Clark County Civil Defense Cave, a lava tube used over time by animals and people; the Spencer Rock House, built of fieldstone for C.W. Hardy and later listed on the National Register of Historic Places; the Spencer Opal Mine, the only public rock-digging opal mine; and the Camas Meadows battlefield, where in 1877 Nez Perce warriors stole General Howard's mules and horses and fought pursuing cavalry, leaving basalt rifle pits still visible today.