What Is a Sea Monster? What Sailors Saw and What They Made of It
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
Every seafaring culture describes creatures in the water that swallow ships, coil around masts or rise from the depths. Some of those accounts are pure story, some are genuine animals described by people with no framework for them, and separating the two is more interesting than dismissing the whole category.
The recurring figures
Certain forms appear repeatedly across unconnected traditions. The vast serpent or coiling creature is nearly universal, appearing as the Norse world serpent encircling the earth, as Leviathan in Hebrew tradition, as sea dragons in Chinese and Japanese accounts and as countless local serpents. The many-armed grasping creature appears in Greek accounts, in Scandinavian descriptions of the kraken and in Caribbean and Pacific traditions. The creature so large it is mistaken for an island recurs from Arabic and European sources to the medieval bestiaries. The whirlpool or devouring mouth appears as Charybdis and its equivalents. Malevolent transforming beings that lure sailors, including mermaids and their many analogues, occupy a separate category concerned with temptation rather than size. The consistency across traditions partly reflects shared experience of the sea, partly reflects transmission along trade routes, and partly reflects the limited number of ways to imagine something dangerous and unseen.
The animals behind the stories
Several accounts have plausible identifications, and a few have been settled:
- •Giant squid, which reach over ten metres including tentacles, were known only from carcasses and sucker scars on sperm whales until photographed alive in 2004 and filmed in 2012, and are the best candidate for the kraken
- •Oarfish, a ribbon-shaped fish reaching eight metres or more, which swims vertically and appears at the surface when dying, and which accounts convincingly for sea serpent reports
- •Basking sharks, whose decomposed carcasses lose the jaw and gill structures and leave a small head on a long neck, producing the repeatedly reported carcasses that appear to be plesiosaurs
- •Whales generally, including surfacing backs mistaken for islands and blows and tails misread at distance
- •Walruses and seals, whose heads at a distance produce reports of humanoid faces in the water
- •Sunfish, manta rays and whale sharks, all of which are genuinely strange to an observer with no prior knowledge of them
Why maps carried them
Sea creatures on early maps were not idle decoration. They marked known hazards in a visual language, indicated regions where information was poor, advertised the cartographer's sources and skill, and filled space that would otherwise look like ignorance. Several were copied directly from natural history texts and represent attempts at real animals, so a map creature may be a whale drawn by somebody who had read about one rather than an invention. The phrase about dragons at the edges is largely a modern invention, appearing on only one known surviving object rather than being a convention. As exploration progressed, the creatures retreated to the parts of the ocean still unvisited and then disappeared from maps entirely, which tracks the growth of knowledge rather neatly. The equivalent today is the acknowledgement that a very large fraction of the deep ocean remains unsurveyed at any useful resolution, which is a factual statement of the same thing the creatures represented.
What is still being found
Treating the category as closed would be a mistake, since large marine animals continue to be discovered. Several whale species have been described in recent decades, including one identified in 2021 and others distinguished by genetic work from populations previously lumped together. The megamouth shark, a filter-feeding shark reaching over five metres, was unknown to science until one was caught accidentally in 1976, and fewer than three hundred have been recorded since. Colossal squid, larger and heavier than the giant squid, was described in the twentieth century and filmed alive only in 2025. Deep-sea exploration produces new species routinely, and new observations of known ones regularly overturn assumptions about behaviour and size. That ongoing record is the honest answer to the question of whether such creatures exist: the specific legends generally do not describe real animals accurately, and the ocean has repeatedly produced animals that would have sounded like legends before they were caught.
The takeaway
Vast serpents, many-armed grasping creatures and beasts mistaken for islands recur across unconnected traditions, and several map onto real animals described without a framework. Oarfish account well for sea serpents and decomposed basking sharks for supposed plesiosaur carcasses. Map creatures marked hazards and gaps in knowledge, and the dragons at the edges phrase appears on one surviving object rather than being a convention.