Formation of the Sea of Japan (26 million years ago - 10 million years ago)
From a paradise where alligators lived, to the bottom of the sea
When volcanic activity began around 26 million years ago, the Oki Islands and the Japanese Archipelago slowly began to separate from the Eurasian supercontinent. As the land was stretched out and elongated, parts of it sank, and in the depression that formed water gathered, leading to the development of a lake environment. What lived here were the alligators and other life forms that were discovered millions of years later as fossils. At this time, Oki was at the bottom of the lake that these creatures swam in.
As the separation from the mainland continued, the land split and let in sea water, and the lake metamorphosed into the ocean. This is the formation of the Sea of Japan. What tells us about this process are shark tooth fossils discovered in a strata that formed around 16 million years ago. This evidence of sharks, which are deep-sea marine life, having been in this area tells us that Oki was, at this time, deep at the bottom of the sea.
Starting with plate movement that began around 26 million years ago, the Japanese Archipelago, including the Oki Islands, gradually began moving away from the Eurasian supercontinent. A large lake formed in the area between Japan and Eurasia, and what became Oki was at the bottom of this lake.
The splitting continued, and sea water entered into the lake. This is the birth of the Sea of Japan. At this time, Oki was deep at the bottom of this sea.