Guida
Located in Bungo-ono City, Oita Prefecture, the Inazumi Underwater Limestone Cave is a geological marvel formed during the Paleozoic era, approximately 300 million years ago. The cave's current form was shaped by the massive eruption of the Aso volcano about 300,000 years ago, which caused the area to be submerged under pyroclastic flow deposits. This unique history has created a world-class landscape featuring underwater stalactites, coral stones, bell holes, and helictites. The cave is well-maintained for tourism, featuring sturdy metal handrails and non-slip pathways, making it safe to explore. With a constant temperature of approximately 16°C throughout the year, it serves as an excellent natural cooling spot during the hot summer months.
The history of this cave is incredibly ancient, dating back to the Paleozoic era. The landscape changed drastically about 300,000 years ago when the eruption of the Aso volcano filled the Nakatsu Mure River canyon with over 70 meters of pyroclastic flow deposits, effectively submerging the cave. Over many millennia, much of this sediment was eroded, leaving the upper half of the cave exposed to the air. This natural process has allowed a unique coexistence of geological formations shaped by both water and air. Furthermore, the water within the cave is part of a natural circulation system, with groundwater filtering down from the Shiratani riverbed located approximately 4km upstream.
Visitors can witness a breathtaking, mystical landscape illuminated by strategic lighting. The contrast between the underwater stalactites and the formations exposed to the air creates a one-of-a-kind visual experience. The pathways are lined with blue lighting that highlights the water surfaces and rock textures. Key geological features to look for include underwater stalactites, coral stones, bell holes, and helictites. Because the paths are well-maintained, you can focus on observing the intricate structures of the cave safely while walking.
Since the cave maintains a stable temperature of 16°C, it is a great destination regardless of the season. It is particularly recommended during the summer to escape the heat. While the facility remains open during the New Year holidays, please note that certain activities, such as the outdoor sauna, have specific start dates (e.g., from January 6th). The lighting effects inside the cave create a different atmosphere depending on the time of day, offering a beautiful play of light and shadow.
Beyond exploring the cave, the site offers various facilities and cultural spots. You can visit the Hotori Garden, featuring koi and duck ponds, or explore the local art galleries and the nostalgic