It's the cold one.
Known as a mizuburo 水風呂, these unheated plunge pools are a familiar sight in Japanese sentō, super sentō and sauna facilities. While they may look daunting, they're an important part of modern Japanese bathing culture, particularly for sauna users.
The idea isn't to endure icy water for as long as possible. Instead, a short dip in the mizuburo after leaving the sauna helps cool the body before resting. Many people then spend a few minutes sitting quietly, allowing their body temperature and breathing to settle. This cycle of sauna, mizuburo and rest has become the foundation of Japan's thriving sauna culture.
If you've heard the Japanese word totonou, this is where it comes in. Many sauna enthusiasts describe reaching totonou after completing several rounds of heating, cooling and relaxing.
Unlike the hot baths, a mizuburo isn't somewhere to linger. Even thirty seconds to a couple of minutes is enough for most people, especially if it's your first time. As with every part of the bathhouse, it's perfectly acceptable to go at your own pace or skip it altogether.
From hot springs and saunas to cold plunge baths, every part of a Japanese bathhouse has its own purpose.
Explore more bathing rituals and traditions in the Yū Journal.