In Norse mythology, Jörmungandr, also known as the Midgard Serpent or World Serpent, is an unfathomably large sea serpent or worm who dwells in the world sea, encircling the Earth (Midgard) and biting his own tail, an example of an ouroboros. As a result of it surrounding Midgard it is referred to as the World Serpent. Jörmungandr releasing its tail is one of the signs of the beginning of Ragnarök.
Thor lifts Jormungandr, disguised as a cat. (Lorenz Frølich)
Thor's fishing trip depicted on the Altuna Runestone, one of the few confirmed Viking Age depictions of Jörmungandr.
Thor fishing for the Midgard Serpent in an illustration from an 18th-century Icelandic manuscript
Thor Battering the Midgard Serpent (1790) by Henry Fuseli
A sea serpent is a type of sea monster described in various mythologies, most notably Mesopotamian (Tiamat), Judaeo-Christian (Leviathan), Greek, and Norse (Jörmungandr).
A sea serpent.―From Magnus, Olaus (1555). History of the Northern Peoples.
"The Great Sea Serpent (according to Hans Egede)".―Engraving c. 1843, signed by artist James Hope Stewart.
"Supposed Appearance of the Great Sea-Serpent, From H.M.S. Plumper, Sketched by an Officer on Board", Illustrated London News, 14 April 1849
Sea serpents, Ama Temple, Macao