Omijima Island located in Kitanagato-Kaigan Quasi-National Park in the northwestern part of Yamaguchi Prefecture is an island with an area of 18.8 sq km and a circumference of 40 km. It was nationally designated as a Place of Scenic Beauty and a Natural Monument in 1926.

The coastline in the northern part of the island is known as “the Alps on the Sea” because there are a diverse variety of bluffs, caves, stone pillars and oddly-shaped rocks. The sightseeing boats from Senzaki Harbor on the mainland provide a close view of the scenery including Bat Caves in a bat-shaped huge rock in the sea and Gold Cave, inside of which is shining gold.

The rock strata creating this eroded land features are composed of solidified lava that flew out at the eruption about 100 million years ago. Then the Sea of Japan emerged about 15 million years ago, and the rock mass was repeatedly eroded by raging waves and gales of the sea to form these natural artworks.