The oceans of the world have always had an air of mystery. About 71% of the Earth's surface is covered by water, and until the 20th century no one had plumbed its depths. Even today the bigger seas and oceans remain a largely unexplored frontier, with new species being discovered every year. Thus it comes as no surprise that countless legends have arisen of strange creatures lurking in the depths. What follows is just a sampling of these stories, including sightings of unusual sea creatures by experienced witnesses in the 19th, 20th, and even 21st centuries.

The idea that there might be large species still swimming in the oceans that haven't been classified by marine biologists isn't as far-fetched as it might sound. For example, the megamouth shark (Megachasma pelagios) was entirely unknown to science until a US naval vessel off the coast of Hawaii accidentally skewered one on its anchor. The megamouth shark is a deep-water shark that can reach up to 18 feet in length, making it quite a large creature to go unnoticed for so long. In fact, since its accidental discovery, fewer than a hundred specimens have been spotted and the species had only been filmed three times.

The Peruvian beaked whale (Mesoplodon peruvians) was discovered the same year and not formally described until 1991. Very little is known about this small species of whale that has only been seen off the west coast of Central and South America. Even more poorly known is the spade-toothed whale (Mesoplodon traversii), which was first documented in 1872 thanks to the discovery of a partial jaw in New Zealand. It wasn't recognized as a distinct species until a partial skull was found in 1986, and no complete body was found until two specimens washed ashore in New Zealand in 2010.