Mammoth Cave, Kentucky

This is one of the weirdest natural wonders in the US that’s said to be both incredibly grand and unusually gloomy. It’s the longest-known cave system on the planet, and over 400 miles of it have been explored. It is even a World Heritage Site.

And while humans have been known to have been studying it for thousands of years, it remains an unusual place to visit. More than 400 miles of winding rock tunnels and vast chambers of Mammoth Cave National Park have been explored, but many more remain untouched.

If you choose to visit, you can descend into the earth and explore the rare beauty and secrets of the cave for yourself on a guided tour. Most range from 30 minutes to five hours.

But that’s not all! Above ground, nature lovers can enjoy horseback riding, hiking, ziplining, and rock climbing. Plus, kayaking and canoeing on the Green and Nolin Rivers are also available, and they flow right through the park.