Mammoth Spring is a large, first magnitude karst spring that arises in the Ozark Plateau within the state of Arkansas. It is the largest spring in Arkansas and the third-largest spring within the Ozark Plateau region behind Big Spring and Greer Spring. Mammoth Spring is the seventh-largest natural spring in the world. The spring's outlet pool is contained entirely within Mammoth Spring State Park. The park is located in the town of Mammoth Spring, which lies in extreme north-central Arkansas. The outlet pool is adjacent to US Highway 63, only 500 ft. south of the Missouri border, and it can be readily seen from the highway. The spring's large discharge volume rushes a few hundred feet down the outlet channel before merging with the Warm Fork of the Spring River to form the Spring River. The cold water that emerges from the spring creates excellent trout fishing conditions and ample flow for water sports on the Spring River. Mammoth Spring was declared a National Natural Landmark by the Department of the Interior in June 1972. Description Precipitation that falls in southern Missouri seeps into the water table and flows through a vast system of passages and cavities. These cavities form an underground river that ultimately emerges on the surface at the town of Mammoth Spring, Arkansas. The emerging water forms a 10-acre spring pool that drains over a high stone dam. The spring generates a harmonic mean flow of about 9 m³/s (322 cubic feet per second). The water emerges at a constant 14 °C (58 °F). The spring itself cannot be viewed at the Mammoth Spring site because its mouth is more than 21 m (70 ft) below the surface of the large spring pool. Nine miles northwest of Mammoth Spring, visitors can see a portion of the underground river that feeds the spring at a collapsed cave in Grand Gulf State Park in Missouri. The remains of a portion of the cave are now a 40 m (130 ft) deep chasm with a natural bridge over it. Dye tests have proven that the water flowing through the 40 m (130 ft) chasm at Grand Gulf exits through the existing cave and emerges at Mammoth Spring. Industrial use The spring was used to power a gristmill from the times of the earliest European settlement. The Mammoth Spring Milling Company constructed a dam and a water-powered gristmill just downstream of