Doing some research in an old newspaper file, I ran across one of the fascinating tidbit fillers of news from here and there. This one reported a volcano in Nebraska. Originally noted by Lewis and Clark, the region was said to be held sacred by local Native peoples and the 'mound' stood some 180 feet and was seen to smoke, flame, and otherwise give the appearance of a volcano. Earthquakes in the 1870's set nerves on edge as did the fact it appeared to belch fire and heat. Subsequent floodings of the nearby river destroyed it and as the areas around it died up, the story too was largely lost. Today, it is believed the 'volcano' was actually the product of chemical reaction. In a 1901 newspaper it was explained: "The volcano is believed to be due to the peculiar formation of the rock, which is of carbonate of lime mingled with innumerable crystals of bisulphate of iron or iron pyrites. The decomposition of the component parts is credited with bringing about the violent chemical action."
Read more here and here.