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Aso-Kuju NP, Oita
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Nearby Sujiyu has a geothermal feast beyond eggs: “Gokuraku Onkei,” a whole chicken steamed with vegetables inside, was remembered as special-occasion food in the postwar years of scarcity.
Researchers collected hot-spring water and sediment at Komatsujigoku and isolated 3 bacteriophages—viruses that infect bacteria. All 3 were filament-shaped, a microscopic surprise beneath this steaming landscape.
Komatsujigoku’s steaming field stretches about 350 m along the stream and 70 m wide. Researchers measured about 170 points 50 cm underground; one spot reached 98.4°C, nearly boiling below the surface.
The acidic bubbling water has an unexpected source: a geochemical study concluded it is rain-derived surface water heated by geothermal steam—not deep acidic water rising from the nearby Hatchobaru field.
At Komatsujigoku’s egg-steaming spot, visitors can cook brought-from-home eggs in about 8–10 minutes. The local tourism association says they come out with a gentle natural saltiness.
The approach changes color before the steaming mud pools appear: sulfur crystals cling to the rocks, while hot-spring minerals stain patches of the mountainside red.
July 2020’s torrential rain damaged Komatsujigoku’s access path, and inspections found cracks in the ground. After 5 years of closure and repairs, the site reopened on July 19, 2025.
