Rather than a natural cavern, it is a 100-year-old flooded limestone mine transformed into a subterranean waterway, you can kayak in. Expect to spend about 1.5 hours. Children must be at least 5 years old to participate in the kayaking tours. The boat tour allows children as young as 3. : These tours are incredibly popular and frequently sell out weeks or months in advance, so booking ahead via Gorge Underground or SUP Kentucky is crucial.