Case No. #1482
The Voices from Below Beppu's Mud Hells
Summary
Between 2018 and 2022, the Bureau received repeated reports from tourists and facility staff at Beppu's famous "Jigoku Meguri" (Hell Tour) area describing voice-like sounds emanating from below the mud pools and fumaroles. The sounds intensified under specific meteorological conditions. During a 2020 acoustic investigation, a specialist recorded two instances of sounds with linguistic tonal patterns unexplainable by geothermal gas alone. A geological survey subsequently found an unexpected subsurface cavity beneath the site.
Details
Beppu's Jigoku Meguri is a collection of active geothermal features — mud pools, boiling ponds, and fumaroles — developed as tourist attractions. The volcanic character of the area has been feared and venerated by local communities since antiquity, as names like "Blood Pool Hell" and "Demon Mountain Hell" reflect.
The first report, in June 2018, came from a facility staff member at one of the mud pool sites. They described hearing a moaning sound from beneath the pool edge, initially interpreted as mechanical vibration but returning repeatedly with a quality that suggested human vocal inflection. Two similar tourist reports from that summer prompted the Bureau to open a file.
In 2020, an acoustic specialist conducted a three-day measurement session at the site. Geothermal gas bubbles breaking at the mud surface were found to produce sound in the 100–200 Hz frequency range — which overlaps with the lower register of human speech. It was theoretically demonstrated that the geometry of subsurface cavities could produce resonance that generates inflection patterns resembling speech.
However, during the investigation, the specialist personally recorded two instances that appeared distinctly speech-like in Japanese. Acoustic analysis confirmed that these instances had tonal structures more consistent with language patterns than could be attributed to natural geothermal gas emission. The specialist noted: "I cannot completely rule out sequential acoustic coincidence, but the occurrence of two such instances in the same session is worth documenting."
A subsequent geological survey detected an unexpected cavity approximately ten meters below the measurement site, with a volume estimated at over 100 cubic meters. A hypothesis that geothermal gas passing through this cavity produces specific resonance effects is currently being modeled. Weather-correlated monitoring continues.
Timeline
- June 2018Facility staff member reports moaning-like sounds from beneath the mud pool edge. First report filed.
- Summer 2018Two tourist reports of similar sounds, both during high-humidity, overcast conditions.
- 2020 surveyAcoustic specialist confirms geothermal gas produces 100–200 Hz sound — overlapping with human voice frequencies.
- 2020 survey (during)Specialist personally records two instances of sounds with measurable linguistic tonal patterns.
- 2021Geological survey finds unexpected cavity (~100 m³) approximately 10 m below the site. Resonance hypothesis proposed.
- PresentResonance simulation ongoing. Weather-correlated monitoring continues.
Witness Testimony
Facility staff (female, 40s)"Standing at the pool edge, I heard something from below that sounded like a voice. It had the quality of moaning, or maybe speaking. At first I thought it was a pipe. But pipes don't speak. It unsettled me enough to step back."
Tourist (male, 30s)"I put my ear close to the ground and I could hear it clearly — something like a human voice. I couldn't make out words, but the intonation felt intentional. My family said they couldn't hear it."
Acoustic specialist (male, 50s)"The general data looks like irregular noise. But those two instances had a structural organization that looked linguistic. Natural sound shouldn't produce that. I couldn't confirm it, though. This is a difficult case."
Kyu's Analysis
Geothermal gas creates sound. It does not create language patterns. The subsurface cavity may be channeling something.
