Photo: Opqr · CC BY-SA 3.0 · Wikimedia Commons
Hirugami Onsen is a hot spring town in Nagano, Japan. It measures pH 9 — strongly alkaline. Officially the water is classed as sulfur spring.
The classic rotten-egg smell and milky white water. It can tarnish silver jewellery, so take it off.
Figures from the town's Japanese Wikipedia article.
35.4547, 137.7181 — open the map
Planning a first soak? Read how an onsen stay works, and what to do if you have tattoos.