Mammoth Cave

Declared in 1941, it protects the longest known cave system on the planet, with over 686 km of passage mapped so far and new connections found every few years, which suggests its true extent is still not fully known. It was explored and used by indigenous peoples more than 4,000 years ago — mummies naturally preserved by the cave's mineral salts have been found — and during the War of 1812 potassium nitrate was mined here to make gunpowder. It is also one of the last refuges of the grey bat, critically endangered through white-nose syndrome, a fungal disease that has decimated bat populations across North America since 2006.