Images of strange inhabitants of the deep sea are one of the most remarkable elements characteristic of geographical maps of the Middle Ages and the Renaissance. It is surprising that such images have never before been the object of detailed and serious study. The subject of the study not only plays an important role in the history of cartography, fine arts and animal studies, but also opens the door to the world of unknown geography and allows us to trace how Europeans' ideas about the World Ocean changed. Moreover, the sea monsters depicted on ancient maps can serve as a key to our understanding of the sources, connections and methods of the cartographers who had a hand in their creation. The author of this beautifully illustrated monograph analyzes the most important examples of depictions of sea monsters on European maps, starting with the tenth century, when images of outlandish sea creatures first appear on graphic pictures of the world, and ending with the sixteenth century. This book, without a doubt, will become a reference book on the subject for many years.