Orcas attacking a whale, from Carta Marina (1539)
From Ancient Origins by April Holloway
Until a few years ago, no serious consideration had been made of the many and varied representations of monsters found on world maps from the 10th century through to medieval and Renaissance times.
Yet they made so many appearances for a reason.
These monsters of the deep had caused concern – indeed struck fear into – sailors around the globe.
Although some of the images seem fantastic to the modern world, most of the creatures had some basis on true encounters, and their depiction on maps are a great example of how mythology and folklore can evolve from real events.
Merian, Matthaeus, "America nouiter delineata," 
Like many cartographers of the era, Matthaeus Merian filled his maps' blank spaces with frightening creatures, like this sinister seamonster.
During the Age of Exploration, sailors provided natural philosophers and cartographers with firsthand accounts of the unfamiliar animals and people beyond the horizon.
Their experiences in this “New World” were informed by folk tales, biblical lore, and racial and cultural biases (not to mention the anxiety of sailing into unknown waters), and as a result, many of the creatures they encountered were interpreted as terrifying monsters.
Merian took this theme a step further, and surrounded his title cartouche with ominous skulls and bones to underscore the mysteries and dangers he and Europeans of his time associated with the New World and its surrounding oceans.
His message can be loosely interpreted as, “Beware, explorers and sailors, or these skulls might be yours!”
In 2013, the British Library released a book which took the study of these creatures seriously and offered a full and detailed account of the menacing artwork appearing on these maps.
Chet Van Duzer's " Sea Monsters on Medieval and Renaissance Maps" (British Library, 2013) catalogues a variety of examples of ‘sea monsters’ which cartographers had seen fit to include in previously unchartered areas of the world, issuing possible warnings of the creatures that had been or might be encountered in certain ocean territories.
Although it has been thought that the inclusion of these mythical creatures were simply the results of illustrators’ over-zealous artistic license and overactive imagination, many of the ocean’s creatures, such as whales, sharks, walruses and squid would have rarely been seen, and would have been viewed as monsters in medieval and Renaissance times.
Taken from the vignettes on Olaus Magnus's Carta marina, Basel c.1544 (Public Domain)
"The creatures look purely fantastic.
They all look like they were just made up," Van Duzer, a map historian at the Library of Congress, said in an interview concerning his book.
"But, in fact, a lot of them come from what were considered, at the time, scientific sources." For example, it was quite usual for the encyclopedias of the time to contain reference to strange looking terrestrial-aquatic-hybrid animals and mapmakers just took some poetic license in depicting them.
In his book, Van Duzer, who was a 2012 Kluge fellow at the Library of Congress, charts the origins of sea monsters from "mappa mundi," medieval European maps of the world; nautical maps; and Ptolemy's Geography, a treatise by the Greco-Roman mathematician and scientist Claudius Ptolemy, which contained an atlas of the known world during the second century.
The Kraken is but one example of a real sea creature being transformed into a beast of legend.
It is first mentioned in the Örvar-Oddr, a 13th century Icelandic saga.
In Scandinavian mythology, this gigantic sea creature was said to be 1 mile long.
It was depicted as great beast that would attack ships and was so huge that its body could be mistaken for an island.
St.Brendan's ship on the back of a whale.
From by Chet Van Duzer and published by the British Library:
'Sea Monsters on Medieval and Renaissance Maps' (CC BY 2.0)
The Kraken was also made it into the first edition of Systema Naturae , a taxonomic classification of living organisms by the Swedish botanist, physician, and zoologist Carolus Linnaeus.
Here the Kraken was classified as a cephalopod, designating the scientific name Microcosmus marinus.
The myth of the Kraken is believed by historians and scientists to relate to the real world giant squid, which can reach 18 meters in length and has been rarely seen due to its normal habitat being deep in the ocean.
Map by Abraham Ortelius Flemmish Cartographer (Public Domain)
By tracing the depictions of sea monsters throughout the centuries, Van Duzer presented an evolution from a world full of dangers lurking in distant oceans where gigantic octopuses and whales drag ships and sailors into the sea, to 17 th century maps showing ships exerting dominion over the beasts of the ocean.
Eventually, the beasts disappeared from maps altogether.
The take away from Van Duzer’s fascinating depiction of sea monsters is that mythological stories and legends of the past, however fanciful they seem, often stem from real life events or experiences.
Many of the stories from our ancient ancestors evolved from real events that were portrayed according to the understanding and knowledge of the time.
They may have become exaggerated and stray far from reality (any creature 1 mile long is stretching it) but the origin often hails from a seed of truth.