Dōgo Onsen is arguably the oldest and most famous onsen (hot spring) in all of Japan. The establishment dates back over 1500 years and has served as a creative muse to many an author and classical poet. Heck, even the imperial family has their own private bathing chamber reserved year-round at Dōgo. The main building of the onsen was also the inspiration for the spirit bathhouse featured in Miyazaki's Academy Award winning animated film Spirited Away. I've included the following picture for comparison.