Mammoth Cave

Mammoth Cave holds the record for the longest known cave system in the world, with more than 400 surveyed miles of passages, and cavers continue to map new sections almost every year. The scale is difficult to communicate: the next-longest cave system on the planet isn't even half this size, and geologists believe there may be hundreds of additional miles still undiscovered beneath the surrounding sandstone caprock.

Above ground, the Green River carves through forested hills dotted with sinkholes, and a small cable ferry, one of the last of its kind still operating anywhere, still carries cars across the river inside the park. Below ground, ranger-guided tours range from an easy elevator-accessed walk to strenuous, headlamp-lit crawls through undeveloped passages, all ticketed and prone to selling out by midday in the summer.

Come for the scale of what's mapped. Stay for the fact that no one actually knows how much more of it there is. Read the story, book your cave tour ahead, and when you leave, collect the stamp.