The Eboshi Islands are located about 1 km west of Tobishima Island. This group of islands features many pillar-like structures called columnar joints, which formed from submarine volcanic activity when the Sea of Japan was expanding. The lava that erupted cooled and hardened slowly, resulting in columnar joints. Because of its blackish color, the rock here is often mistaken for basalt, but it is actually andesite—the same rock that makes up Mt. Chōkai.

To protect the columnar joints and the breeding ground for the black-tailed gull (Larus crassirostris), this area is designated as a special protection zone of Chōkai Quasi-National Park. However, you can climb atop the columnar joints, or swim off the Gusu Shore, which is made up of andesite pebbles.

These columnar joints are nicknamed “”lumber rocks”” by the islanders. According to legend, Kōbō Daishi attempted to build a bridge from Sakata to Tobishima Island, but the attempt failed, and the lumber he was using turned into rocks. The sea surrounding the Eboshi Islands is very clear, making it a popular spot for scuba diving and rock fishing.