The intense blue color is a result of light scattering from the thermal water's chemical composition, specifically colloidal silica.
The heat from the steam is used to grow giant Victoria Amazonica water lilies in an on-site greenhouse.
The pond reaches depths of approximately 200 meters.
The facility manages its own dedicated thermal source, separate from the other Jigoku sites.
Umi Jigoku, or Sea Hell, is the largest of the eight hot springs comprising the Beppu Jigoku Meguri. The pond features a distinctive cobalt blue color caused by high concentrations of dissolved iron and silica. The water temperature reaches 98 degrees Celsius, making it strictly for observation rather than bathing. The park grounds feature a landscaped garden, a secondary reddish-orange thermal pond, and a greenhouse heated by geothermal steam. Visitors can use a free footbath fueled by the park's natural spring water. The on-site shop and restaurant specialize in regional items including steam-cooked snacks and puddings. The facility is part of a designated National Place of Scenic Beauty. It has been a prominent thermal attraction in Beppu since its formation approximately 1,200 years ago following a volcanic eruption.
The raised viewing platform overlooking the blue pond provides the clearest angle of the water and the surrounding steam plumes.
Bring a small towel for the footbath, as the facility charges for disposable ones.
Try the Jigoku-mushi pudding, which is slow-cooked using the natural thermal steam.
Visit the red pond (Chinoike-jigoku-style) located within the same park grounds to compare the thermal chemistry.
Do not mistake the display ponds for swim-ready hot springs; entering the main blue pond is strictly prohibited.
Do not touch the water in the main pools as it is near boiling; wear comfortable shoes for walking the garden paths.
