Arkansas and Missouri could be at odds with the true ownership of Mammoth Spring, the largest spring in Arkansas, and the seventh-largest in the world. The mouth of the spring is in Arkansas (only 500 feet from the MO/AR border), but the recharge area belongs to Missouri. For now, the states are in agreement, the second-largest spring in the Ozarks belongs to Arkansas.
Almost ten million gallons of water pump out of Mammoth Spring every hour. A hydroelectric dam harnesses the massive power of this water. In 1957, the dam, spring, and spring-fed fish hatchery site were acquired and made into an Arkansas State Park.
Now a National Natural Landmark, the spring channels into a lake. The lake runs into Spring River, a mecca for trout fishermen with its chilly 58 degrees Fahrenheit water. The park is free and there are numerous trails to see it all. But Mammoth Spring is also viewable from Highway 63. A portion of the underground river that feeds into the spring is viewable at Grand Gulf State Park in Missouri.