"Friedrichsbad, a nude co-ed bathhouse in Baden-Baden, Germany, built in 1877, assembly-lines guests through 17 stations of relaxation, including a 154°F hot-air room and an arctic plunge pool. It's known for its soothing thermal waters with lithium and trace amounts of arsenic. Mark Twain once said of his visit that you lose track of time within 10 minutes and track of the world within 20."