It's been 75 years this week that the cave discovered by a farmer looking for his lost pigs in southeastern Minnesota opened for tours.
Since then, the vast cavern containing massive stalactites, prehistoric fossils and a 60-foot waterfall has become a local landmark.
Today, about 25,000 people tour Niagara Cave each year. The anniversary is being celebrated Monday with a town picnic in Selvig Park in Harmony.
The husband-wife team of Mark and Jenny Bishop have owned the cave since 1995. After buying it, the Bishops made the cave easier to tour and replaced the outhouses with indoor bathrooms.
These days, visitors can pan for gemstones and fossils using a mining sluice and there's a wedding chapel where more than 400 couples have said their vows.