Photo by 663highland, via Wikimedia Commons (CC BY 2.5)
🇯🇵 Japan · Oita
♨ Temperature
98°C
⛰ Altitude
160 m
⚗ pH
6.8
💧 TDS
890 mg/L
🌊 Flow rate
8 L/s
📅 Established
1200
✓ Verified on 2026-07-22
Umi Jigoku, meaning Sea Hell, is the most photographed of Beppu's jigoku — a series of geothermal pools that have been tourist landmarks in Oita Prefecture since the early 20th century. The pool's remarkable cobalt blue colour results from dissolved ferric sulphate, which scatters light in the same way that ocean water does. Despite reaching 98°C, the pool has a near-neutral pH, making it compositionally different from the acidic springs found elsewhere in Japan. A volcanic eruption over 1,200 years ago created the underground hydrothermal system that still feeds the pool today.
| Ferric Sulphate | 280 mg/L |
| Chloride | 310 mg/L |
| Sodium | 190 mg/L |
| Silica | 95 mg/L |
The eight jigoku of Beppu have been recorded in local chronicles since the Heian period. Umi Jigoku was designated a National Scenic Site by the Japanese government in 2009.
The tradition of jigoku mushi — steam-cooking eggs, corn, and sweet potatoes over geothermal vents — is practiced at several of the Hell sites and has become a defining culinary experience of the region.
Umi Jigoku is part of a UNESCO Global Geopark candidate zone covering Beppu's volcanic hydrothermal system. Spring output and chemical composition are monitored quarterly.