“HIC SUNT DRACONES.”

This phrase translates from the Latin as “here are dragons.” It is etched on the eastern coast of Asia on one of the oldest terrestrial globe maps, the Lenox Globe, dating to 1510. Though the phrase itself is found on only one other historical artifact—a 1504 globe crafted on an ostrich egg—depictions of monsters and mythological beasts are common on early maps. They mostly crop up in unexplored reaches of the oceans, warning would-be explorers of the perils of these unknown territories.

One of the most famous of these maps is Olaus Magnus’ Carta Marina, drawn between 1527and 1539. Magnus (1490-1557) was the Catholic archbishop of Sweden and a prominent historian. His travels brought him farther north than any of his contemporary European intellectuals, lending a great deal of perceived credibility to his accounts and publications. Carta Marina is a detailed map of Scandinavia—one of the oldest ever created—and it depicts the Norwegian Sea so teaming with monsters that it would seem impossible to escape these waters uneaten. In 1555, Magnus published Historia de Gentibus Septentrionalibus (“A Description of the Northern Peoples”), which not only related the history, customs, and beliefs of the Scandinavian people, but also reprinted and described the creatures found on Carta Marina. His standing and reputation secured the widespread acceptance of his stories.

Magnus’ descriptions and drawings were copied repeatedly, with little to no modification, for centuries by such historical titans as Edward Topsell, Ulisse Aldrovandi, John Jonstonus and Conrad Gessner, whose Historia Animalium, replete with Magnus’ drawings, is the first modern zoological work attempting to describe all known animals. Such repurposing ensured that these creatures were ingrained in the public mind as truth. And over the centuries, many new monsters were added to the mix.

Where did the accounts of monsters come from in the first place? Were they simply fairy tales invented to scare curious minds and small children? Henry Lee, who wrote extensively on sea creatures and monsters, emphasized that many classical monsters are not simply pure myth. In his publication Sea Fables Explained (1883), he wrote, “… the descriptions by ancient writers of so-called ‘fabulous creatures’ are rather distorted portraits than invented falsehoods, and there is hardly any of the monsters of old which has not its prototype in Nature at the present day.”

These “distorted portraits” came about in part because by the 1500s extensive oceanic exploration was still limited, and the fauna that called these places home remained virtually unknown. Publications by Magnus and those who copied him represented some of the first attempts to systematically aggregate and describe these animals. More often than not, their information came not from first-hand observations but from sailors’ accounts of mysterious encounters while at sea. Less often, the decomposing remains of a washed-up carcass fueled confidence in the existence of these terrible beasts.

Sailors, or beachgoers who had the misfortune to stumble upon a rotting basking shark, had no experience with such creatures. So they explained them with what they knew well: myths and legends. If they enlivened their accounts, that simply made for a better story. And so an oarfish became a 200-foot-long sea serpent. A giant squid became a blood-thirsty kraken. A manatee became a mermaid. Magnus and others like him gobbled up the stories and published them alongside authentic species. The more the stories were circulated and published, the more likely people were to mistake what they did see for a monster. And the cycle continued.

The atmosphere of the day also fed people’s willingness to believe such tales. The 1500s were rampant with superstition. The Scientific Revolution would not start to make headway until later in the 17th century. There was no division between magic and reality—the two simply coexisted, so there was no reason to doubt mythical beasts. And even when scientists began to embrace the scientific method, they still struggled to reconcile previous beliefs in the supernatural with science. It would take hundreds of years of dedicated scientific study and exploration to overturn classical and common opinion. In the case of some creatures (i.e., sea serpents), sightings and questions of authenticity still remain.

Today we know that the animals that inspired such hair-raising tales as the sea serpent, leviathans and hydra and authenticated stories of mermaids and the kraken are real. They just received some creative embellishments (and sometimes blatant artistic fraudulence) along the way. And in a world just beginning to turn away from superstition, but still inclined to embrace elements of mysticism, it’s not surprising that the tales were accepted. Besides, who doesn’t love a good monster story?

Follow the Biodiversity Heritage Library blog and #bhlMonstersRreal on Twitter all week to get the scoop on the people, books and animals that inspired some of history’s most legendary monsters—including the full stories behind these five incredible beasts: