Give or take—and we're ballparking here—there are three basic levels of hot-water spa-going. Level one: when you have time, say twice a month, you hop into the relaxing, triple-digit temperatures of the spa in your local gym or health club. It's not private and you certainly wish you could get there more often, but you're simply too busy. Level two: you enjoy the hot, swirling water so much you pony up enough cash to install a spa or jacuzzi on your very own back porch. For the super-addicted, however, the only true way to experience a hot-water spa is in the great outdoors. Enter Mother Nature and her thermal springs, where dissolved minerals can do wonders for the health of your skin. Even macaques can't turn down a soak in some of these spas.