One of Japan's Largest Caves in Pyroclastic Flow Deposits
The Mizonokuchi Cave (溝ノ口洞穴) is a cave formed in the pyroclastic flow deposits of the Shirasu plateau, at the head of a side valley of the Mizonokuchi River in Soo City, Kagoshima Prefecture, which in 2021 was designated a Natural Monument.
The Agency for Cultural Affairs describes it as one of the largest caves formed in pyroclastic flow deposits in Japan, 209.5 metres long.
Soo City notes that it was the city's first nationally designated cultural property.
Geology and Formation
According to the Agency, the cave is formed in the Ito pyroclastic flow deposit, partly weakly welded, with an entrance 13.8 metres wide and 8.6 metres high.
At the entrance, the layers can be seen dipping toward the valley: about 1 metre of Osumi pumice fall at the base, about 6 metres of unwelded deposit, about 3.5 metres of welded deposit, and more unwelded deposit above.
The cave formed through groundwater in the Shirasu plateau, which preferentially eroded the unwelded lower part of the deposit; repeated collapse enlarged the cave, while the strong welded layer keeps the roof from falling.
What to Look For
The Agency notes clear traces of gas venting through the hot pyroclastic flow and of erosion by groundwater. Soo City points to holes in the ceiling, called fumarolic pipes, where gas and steam escaped upward.
The city explains that the Ito pyroclastic flow erupted from the Aira caldera about 29,000 years ago, and that inside the cave the deposit can be seen hardened into welded tuff by heat and compaction. As the longest welded-tuff cave, among other reasons, it was chosen in 2013 as one of Japan's hundred notable caves.
Visiting the Mizonokuchi Cave
Soo City notes that local people have long cared for the cave, holding a cave festival with yakko and stick or sword dances on the Sunday nearest the Buddha's birthday, 8 April.
Check directions with the Soo City tourism association. As a Natural Monument, the cave walls must not be damaged and rocks must not be taken. Enquiries: Soo City lifelong learning section, 0986-76-8873.
Q&A
- When was Mizonokuchi Cave designated?
- On 26 March 2021.
- How big is the cave?
- 209.5 metres long, with an entrance 13.8 metres wide and 8.6 metres high.
- How did the cave form?
- Groundwater eroded the lower pyroclastic deposit and repeated collapse enlarged it.
- What are fumarolic pipes?
- Holes in the ceiling where gas and steam escaped upward, according to Soo City.
- Is there a festival at the cave?
- Yes, a cave festival on the Sunday nearest 8 April.
Basic Information
| Name | Mizonokuchi Cave (溝ノ口洞穴) |
|---|---|
| Location | Soo City, Kagoshima Prefecture |
| Designation | Natural Monument |
| Year Designated | Designated 2021/3/26 (Reiwa 3) |
| Quantity | 1 site |
| Dimensions | 209.5 metres long; entrance 13.8 metres wide, 8.6 metres high |
| Owner | Not recorded in the Agency for Cultural Affairs database |
| Public Access | Viewable at the cave |
References
- Agency for Cultural Affairs Database: Mizonokuchi Cave (Japanese)
- https://kunishitei.bunka.go.jp/bsys/maindetails/401/00004126
- Cultural Heritage Online: Mizonokuchi Cave (Japanese)
- https://online.bunka.go.jp/heritages/detail/589460
- Soo City: Mizonokuchi Cave (Japanese)
- https://www.city.soo.kagoshima.jp/kankou_event/rekisibunnka/siseki/mizonokuchidouketsu.html
Last verified: 2026.09.28