The cave for which the city is named is located on the south side of Main Street. Various explanations are given for its unusual name. One is that Native Americans or outlaws hid horses in the cave; another is that an early carriage may have lost a horse when it fell into the opening by accident; a third reason involves the 19th-century use of "horse" as slang for anything big or huge, related to a similar use of the Swedish "hoss". The cave has also been known as "Hidden River Cave", for an underground stream located inside. That stream was used to power a dynamo, and for a while in the late 19th century Horse Cave was the only city in Kentucky apart from Louisville and Ashland to have electric lights. Around World War I, the only air-conditioned tennis courts in the world were located near the entrance of the cave. The stream also provided the town's water supply, but mistaken development caused raw sewage to seep into the water and forced the closure of the cave for fifty years. The stench was so bad in the 1960s that pedestrians typically crossed to the other side of Main Street rather than walk near the entrance and birds would sometimes fall from the sky. The problem has been largely solved in recent years.