Genbudo Park is located about 10 minutes by car from Kinosaki Onsen. It is also worth visiting as a power spot! 
Genbudo Cave Park was formed when magma that flowed from the summit of a mountain during volcanic activity approximately 1.6 million years ago cooled and solidified.
It is said that it was washed away by the waves and revealed itself about 6,000 years ago, and that a cave was created when people removed stones from it. 
The sight of the hexagonal basalt rocks piled up is a sight unlike any other. 

I've been there a few times, and the basalt cave is overwhelmingly impressive. Also, for some reason, the area around it feels cool. It's a relaxing space. 

This park was designated a national natural monument in 1931, and in 1963 it became the San'in Kaigan National Park, a status it retains to this day. 
The hydrangeas at Genbudo Cave are very famous, and the Hydrangea Festival is held every year in late June. You can stroll through the park while admiring the hydrangeas in full bloom, and there are also limited-time events to enjoy.