Mammoth Spring is a very large first magnitude spring. It is the largest spring in the state of Arkansas and the third largest spring within the Ozark Plateau region behind the Big Spring and Greer Spring. It is located in Mammoth Spring State Park, an Arkansas state park, in the extreme north-central part of the state at the town of Mammoth Spring. The outlet pool of the spring is located a just to the east of US Highway 63 (US 63) and roughly 500 feet (150 m) south of the Missouri border. Mammoth Spring's large outlet pool can easily be seen by travelers on US 63. The spring's large volume of water rushes a few hundred feet before merging with the Warm Fork of the Spring River to form the Spring River headwaters. The cold water provides excellent trout fishing conditions in addition to ample water for float trips on the Spring River. It was declared a National Natural Landmark in June, 1972.