Inatsumi Underwater Cave is one of the largest underwater caves in Japan. The cave was formed over a long period of time during the Paleogene period (approximately 300 million years), but was submerged by the great eruption of Mount Aso about 90,000 years ago and became underwater. As a result, a unique and mysterious underwater landscape was created that cannot be seen in land caves.

Various types of stalactites have formed inside the cave, including underwater stalactites, coral stones, bell holes, and helictites.

The water inside the cave is also very clear, allowing you to enjoy a beautiful view like an aquarium.

The temperature inside the cave is around 16 degrees all year round, cool in summer and warm in winter, and you can walk around the 500m one way in about 30 to 40 minutes.

[Trivia]
The cave was only recently discovered, and in 1976, divers from a diving expedition team began diving surveys, discovering the longest underwater cave in Japan, at about 1,000 meters. It all started when a child playing in the river discovered a hole underwater.

[Useful points of concern]
Even though it's an underwater cave, regular visitors are not allowed to dive in it. You can only explore on foot.