This book is devoted to unknown pages of the history of the naval fleet of the late XIX - early XX centuries. The search for the battleship Mermaid that disappeared in the Gulf of Finland, the mysterious explosions of ships during the First World War, the riddles of the most outstanding Russian intelligence officer Anna Revelskaya, the mysterious incident on the Askold cruiser, which influenced the course of the Civil War in the North... These and many other naval mysteries are recounted in a book offered to readers.