Bull Shoals Caverns in Bull Shoals, Arkansas, is a limestone cave formed about 350 million years ago as water dissolved rock and created passages that are still shaped by an underground river. Human use reaches back to about 300 B.C., with evidence of dwelling and association with early Native American groups, including people described as Cliff Walkers. In later eras, local residents used the cave in practical ways: Confederate soldiers reportedly gathered saltpeter there during the Civil War, settlers used it as a natural cooler, and moonshiners hid stills there during Prohibition. Since 1958, it has also been a public attraction in the Ozarks.