Pure hot springs are potent; drink plenty of water before and after your soak.

Hidden in the mountains, this collection of seven inns offers a "secret hot spring" vibe. Tsurunoyu, the oldest, features milky-white water flowing directly from the source into rustic outdoor baths. 3. Beppu Onsen (Oita)

While specific rankings can fluctuate based on seasonal water quality and traveler reviews, the following locations consistently dominate the "Top 13" list for their historical significance and water purity. 1. Kusatsu Onsen (Gunma)