thumb|A sea serpent.
thumb|[[Mosaic of a cetus sea serpent of Greek mythology]]
A sea serpent is a type of sea monster described in various mythologies and religious texts. Famous sea serpents includes: the Mesopotamian Tiamat, Ugaritic Yam and Tannin, Judaic Leviathan and Rahab, ancient Greek Cetus, Echidna, Hydra and Scylla, Vedic Vritra and Surasa, and the Norse Jörmungandr, etc.
Mythology
Mediterranean and Western Asia
The mytheme, the chief god in the role of the hero slaying a sea serpent, is widespread both in the ancient Near East and in Indo-European mythology,
e.g. Lotan and Hadad, Leviathan and Yahweh, Tiamat and Marduk (see also Labbu, Bašmu, Mušḫuššu), Illuyanka and Tarhunt, Yammu and Baal in the Baal Cycle etc. The Hebrew Bible also has mythological descriptions of large sea creatures as part of creation under Yahweh's command, such as the Tanninim mentioned in Book of Genesis 1:21 and the "great serpent" of Amos 9:3. In the Aeneid, a pair of sea serpents killed Laocoön and his sons when Laocoön argued against bringing the Trojan Horse into Troy. Claudius Aelianus in his work On the Nature of Animals mentions a giant sea centipede, which has a tail that is similar to a crayfish and which moves using numerous feet on each side of its body. Guillaume Rondelet mentions a similar imaginary creature called centipede cetacean in his work L'histoire entière des poissons.
Germanic Scandinavia
In Nordic mythology, Jörmungandr (or Midgarðsormr) was a sea serpent or worm so long that it encircled the entire world, Midgard. Sea serpents also appear frequently in later Scandinavian folklore, particularly in that of Norway, such as an account that in 1028 AD, Saint Olaf killed a sea serpent in Valldal in Norway, throwing its body onto the mountain Syltefjellet. Marks on the mountain are associated with the legend.
Natural history
An apparent eye-witness account is given by Aristotle in his work Historia Animalium on natural history. Strabo makes reference to an eyewitness account of a dead sea creature sighted by Poseidonius on the coast of the northern Levant. He reports the following: "As for the plains, the first, beginning at the sea, is called Macras, or Macra-Plain. Here, as reported by Poseidonius, was seen the fallen dragon, the corpse of which was about a plethrum [] in length, and so bulky that horsemen standing by it on either side could not see one another, and its jaws were large enough to admit a man on horseback, and each flake of its horny scales exceeded an oblong shield in length." The creature was seen sometime between 130 and 51 BC.
Norway, 16th century
Swedish ecclesiastic and writer Olaus Magnus included illustrations of sea serpents and other various marine monsters on his illustrated map, the Carta marina. In his 1555 work History of the Northern Peoples, Olaus gives the following description of a Norwegian sea serpent:
Norwegian Bishop Erik Pontoppidan (1698–1764) did not disbelieve the existence of sea serpents themselves, but doubted they would prey on ships and feed on humans, being more cautious-minded in that respect than Archbishop Olaus (of Uppsala). Nevertheless, a number of reports were made by sailors at the time that sea serpents would destroy ships by wrapping the ship in coils of their body and pulling it underwater. Sailors threatened by a sea serpent were said to have thrown large objects such as paddles or shovels overboard in the path of the serpent, hoping that the serpent would take the object and leave without destroying the ship.