Thermae Yu
1-1-2 Kabukicho, Shinjuku
This onsen in the Shinjuku area of Tokyo sources its pristine waters daily from Naikazu in the Izu Peninsula. Pure soaking pools aside, there's an infrared sauna, as well as bedrock baths. The latter involve lying on beds made of rock salt, said to release energizing negative ions. The massages here are completely worth it, rigorous in a hurt-so-good kind of way.