More than 95 percent of the ocean remains unexplored, yet we know so much more about it than our ancestors did just a century or two ago. Imagine how much more frightening the seas must have seemed back then, potentially writhing with all manner of monsters. While scientists are fairly confident that the oceans aren’t hiding any colossal monsters of Cthulu proportions, we truly don’t know what might be lurking in the deep. Here are four sea creatures that were commonly mistaken for ‘sea monsters’ in times past, due to their size or appearance after decomposing.
Giant Oarfish
One glance at the length of the giant oarfish, and it’s clear why people would have seen it as a sea serpent. Slithering through the water like an eel, the oarfish is a species of bony fish that has been officially measured at up to 25 feet in length, though reports put some specimens at twice that size. The largest known specimen weighed 600 pounds. Living at depths of 66 to 1000 feet, giant oarfish don’t often appear where we can see them, but they occasionally surface when ill or dying. In 1860, two men gathering seaweed camp upon a giant oarfish trapped on some rocks, and killed it, taking it for a sea monster.
Frilled Shark
Another serpent-like creature with an even more frightening appearance is the frilled shark, which is considered to be a living fossil. This primitive shark only reaches lengths of about six feet, but it’s still pretty monstrous. It differs from other sharks by virtue of its front-positioned jaws; it has needle-like teeth that help it catch its primary food of soft-bodied squid.
Basking Shark
The second-largest fish in the sea, basking sharks have a pretty unusual appearance when they’re feeding. These plankton-eaters swim around with their mouths wide open, and considering that they can reach up to 40 feet long, that’s pretty intimidating to people who don’t realize these creatures aren’t interested in eating humans. But even more so than its appearance while alive, it’s the way the basking shark looks once dead that once made it seem like a monster. Because its gills rot quickly, it can have the appearance of having a long, narrow neck once it washes up on shore.
Giant Squid
And of course, there’s the giant squid. Despite being the largest invertebrate on earth, the giant squid is still a mystery to science, and we’ve only just recently gotten video of one in its natural habitat. Giant squid have occasionally washed up on beaches or been hauled in by fishermen; in 2004, Japanese scientists captured the first images ever taken of a live specimen, and in 2008, Japan’s National Science Museum brought a 24-foot-long living giant squid to the surface. They have the largest eyes in the animal kingdom, measuring about 10 inches in diameter.

