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Inazumi Underwater Cave

Little knownBarely documented in English. This is the kind of place the site exists for.

Researched and written by Hinato

Inazumiunderwater cavelimestonegeoparkBungo-ono

Photo: ブルーノ・プラス / Wikimedia Commons (CC BY 4.0) · original

Oita's geography does a lot of the work at Inazumi Underwater Cave — the rest is knowing when to show up.

Inazumi is a limestone cave that was drowned. The cavern formed over two hundred million years ago and was flooded roughly 85,000 years ago by the pyroclastic flows of an Aso eruption, which sealed the lower system under water and left the stalactites and columns standing intact beneath it. It was rediscovered in 1976 and is one of very few underwater caves anywhere open to ordinary visitors.

The walking route follows the dry upper cave and then a submerged section where the water is clear enough to see formations several metres down through it. Cave divers have surveyed well beyond the public route and the full extent is still not established. The site also runs a separate dry cave, an emerald-green spring pool and a small museum, and the surrounding Bungo-ono landscape is a UNESCO Global Geopark built entirely on the same Aso deposits.

Visiting essentials

Best season

Year-round; summer for the cool interior | Avoid the days immediately after heavy rain, when the submerged section clouds

How long to allow

1–1.5 hours

Hours

9:00 AM – 5:00 PM (extended in summer)

Closed: Open year-round

Entry fee

¥1,500 adults | ¥1,200 junior and senior high school | ¥900 age 4 to elementary school — confirm current pricing

Getting out there

Directions

Mie-machi Nakatsuru 300, Bungo-ono City. By car from Mie-machi; a car is necessary. Open year-round. Admission ¥1,500 adults, ¥1,200 junior and senior high school, ¥900 age 4 to elementary school — confirm current pricing before travelling. The cave holds a constant temperature around 16 degrees and the floor is wet.

Local knowledge

**Sixteen degrees, all year.** A blessing in August and cold in a T-shirt any other month. Bring a layer. **The water is the point, so go when it is clear.** Heavy rain upstream clouds the submerged section for a day or two afterwards. Clear spells give visibility of several metres straight down onto formations. **Wet floor, low ceilings in places.** Grippy shoes, and mind your head. Not suitable for anyone unsteady. **The dry cave is a separate ticket area.** Ask what the admission covers before paying; the site bundles differently at different times. **Bungo-ono is a geopark, not just a cave.** Harajiri falls, the Chinda falls and the Kanmon and Inazumi limestone all come from the same Aso eruptions. A car and a day covers the set.

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