11.  Seattle Underground, Washington

Following a devastating fire in 1889, Seattle was rebuilt one story higher. Roads were raised and ground floors became basements to lift it out of the boggy ground. New pavements were created and tunnels were left underneath it all. Today you can explore these old subterranean tunnels via guided tours and see what remains including bank vaults, shop fronts, and even an old toilet. 

12.  Mammoth Cave, Kentucky

This mammoth labyrinthine cave network in Mammoth Cave National Park has over 405 miles of documented passages. It’s the world’s longest cave system. Here you can take guided hikes through 10 miles of tunnels. See the famous Frozen Niagara which resembles a waterfall sculpted out of rock. These caves are also home to albino shrimps, endangered bats, and cavefish but book in advance.