Oshakujima Island is located about 1 km west of Tobishima Island and rises 72 m above sea level, higher than Tobishima Island. It is made up of rhyolite, a rock that was formed by submarine volcanic activity when the Sea of Japan was expanding, and that was later uplifted.
This area is designated as a special protection zone of Chokai Quasi-National Park. On the northern side of the island about 5 m above the water, there is a sea cave whose interior looks like the scales of a dragon. This cave has long been revered as a sacred place by islanders and sailors.

Seabirds such as the black-tailed gull (Larus crassirostris), Japanese murrelet (Synthliboramphus wumizusume), and rhinoceros auklet (Cerorhinca monocerata) use this area as breeding grounds; as such, it has been designated as a Natural Treasure. The reason for the dragon-scale appearance of the cave’s rocks is thought to be linked to these birds’ droppings. The phosphorus components in their droppings are thought to have dissolved in rainwater, seeped through the cracks in the rocks, and accumulated in this scale-like pattern.
The banded houndshark (Triakis scyllium) can also be found on the seafloor.

With the formation of the islands and sea caves, the breeding of seabirds, and the marine culture of the local people, this geopark site is a wonderful example of how geology, nature, and culture intersect.