Cray-fish mouths of the sea are very rare in the waters around Japan.
However, they do exist and are known to have existed long before the last ice age.
The oldest known mouth is located off Hokkaido, a mountainous region that is part of the Chukchi Sea.
According to an article in the Japan Times, a Japanese fisherman named Kazuaki Tanaka was searching for a cray-fishes mouth when he stumbled across one of these mounds in Hokkaidō Bay in 2003.
He called the Japanese Coast Guard to find out more and discovered that it was a huge crater, with a deep depression about 3 feet deep.
The crater was made up of a deep watery deposit that is now covered by sediment.
Tanaka has now made the mouth his own, creating a crater with the mouth that he called the Cray Mouth of Japan.
A crater with a Craymouth of Japan is a Japanese discovery in 2003, Japan Times.
Source: Japan Times