Osorezan Onsen Kojigoku Kamanoyu

A raw sulfur spring on the slopes of Mount Osore (恐山), one of Japan's three great "spirit mountains." The hydrogen sulfide is strong enough to smell from a distance, and the spring flows hot enough that the locals sometimes skip it rather than dilute the precious water. There's a basic hut with a tub, plus the possibility of digging your own pooling spot along the bank. About as elemental as onsen bathing gets.