佐久山温泉 is a hot spring town in Tochigi, Japan. The water leaves the ground at 55°C, hot. It measures pH 7.9 — mildly alkaline, the slippery kind. Officially the water is classed as simple spring, chloride spring and sulfur spring.
Mild and low in dissolved minerals. Usually clear and gentle on the skin, which is why these baths are often recommended for first-timers.
Salty water. The salt leaves a thin film on the skin, so you stay warm well after leaving the bath.
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.
Planning a first soak? Read how an onsen stay works, and what to do if you have tattoos.