Salty water. The salt leaves a thin film on the skin, so you stay warm well after leaving the bath.
Clear water with a faintly bitter taste. Known for holding heat.
Strongly acidic, sometimes below pH 2. It stings on cuts and is not for long soaks.
The classic rotten-egg smell and milky white water. It can tarnish silver jewellery, so take it off.
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Planning a first soak? Read how an onsen stay works, and what to do if you have tattoos.