A sea serpent or sea dragon is a mythological sea monster either wholly or partly serpentine.
Sightings of sea serpents have been reported for hundreds of years, and continue to be claimed today. Cryptozoologist Bruce Champagne identified more than 1,200 purported sea serpent sightings. Despite these numerous sightings, no credible physical evidence has been recorded and it is currently believed that the sightings can be best explained as misidentification of known animals such as oarfish and whales. Some cryptozoologists have suggested that the sea serpents are relict plesiosaurs, mosasaurs or other Mesozoic marine reptiles, an idea often associated with lake monsters such as the Loch Ness Monster.
In Norse mythology, Jörmungandr, or "Midgarðsormr" was a sea serpent so long that it encircled the entire world, Midgard. Some stories report of sailors mistaking its back for a chain of islands. Sea serpents also appear frequently in later Scandinavian folklore, particularly in that of Norway.
In 1028 CE, Saint Olaf killed and threw onto the mountain Syltefjellet in Valldal, Norway a sea serpent, the marks of which are still visible  . In Swedish ecclesiastic and writer Olaus Magnus's Carta marina, many marine monsters of varied form, including an immense sea serpent, appear. Moreover, in his 1555 work History of the Northern Peoples, Magnus gives the following description of a Norwegian sea serpent:
Those who sail up along the coast of Norway to trade or to fish, all tell the remarkable story of how a serpent of fearsome size, 200 feet long and 20 feet wide, resides in rifts and caves outside Bergen. On bright summer nights this serpent leaves the caves to eat calves, lambs and pigs, or it fares out to the sea and feeds on sea nettles, crabs and similar marine animals. It has ell-long hair hanging from its neck, sharp black scales and flaming red eyes. It attacks vessels, grabs and swallows people, as it lifts itself up like a column from the water.
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