Inazumi Underwater Stalactite Cave is one of the largest stalactite caves in Japan that is said to have formed 300 million years ago. The cave submerged when Mt. Aso erupted 300,000 years ago and is what it is now. The underwater cave features gorgeous stalactite and is a surreal experience. The cave stays at about 61 degrees Fahrenheit throughout the year and is a popular place to cool down and visit during the summer. A light jacket is recommended as well as closed toe shoes.