This is Fish-Man Island, located 10,000 meters beneath the sea at the Holy Land of Mary Geoise in the Grand Line.
As the mandatory passage point for reaching the New World and the birthplace of most Fish-Men and Merfolk, it is renowned as an "underwater paradise" and a dreamlike haven.
Fish-Man Island resembles a magnificent and dazzling pearl in the deep sea. A transparent bubble-like dome encapsulates the entire island, acting as a mysterious barrier that wards off the immense pressure of the ocean.
Chapter 37