The enigmatic oarfish could be the truth behind ancient tales of sea serpents capable of sinking ships.
Growing up to 36ft (11m) long and swimming like a snake at the ocean’s surface, oarfish match historic descriptions of frightening monsters from the deep.
The giant fish is still capable of sending witnesses fleeing for their lives and sightings in shallow water or incidents of oarfish beaching themselves seem to be increasing.
In reality oarfish are harmless and rarely seen as they are thought to normally live at depths of 500-1000ft (152-305m). But still very little is known about them.
The oarfish family (Regalecidae) contains the longest bony fish in the world (Regalecus glesne) which was first described in 1772 by Peter Ascanius.
Until recently no oarfish had been filmed in their own habitat, the rare footage is shown in the BBC Two programme Nature's Weirdest Events.
The film shows an oarfish swimming vertically, making undulations with its dorsal fin to keep it in a stationary position, which is thought to be for feeding. This contrasts with the snake-like movements they have been documented making in shallow water.
It's like making contact with aliens
“They are gorgeous fish, silver with iridescent blue blotches, bright red fins,” says Rick Feeney, ichthyology collections manager at the Natural History Museum of Los Angeles County.
“In the open ocean they are perfectly adapted to blend in with the water. People are attracted to them when they wash up. It's like making contact with aliens.”
Mr Feeney says oarfish coming into shallow water or beaching themselves are usually rare, localised events. There have been more than 500 recorded sightings since the mid-1700s in locations including the British Isles, Australia, USA, Mexico, Costa Rica and Japan.
In 2013 two dead oarfish washed up on beaches in the USA within a week of each other.
“People were ecstatic,” says Jeff Chace, from the Catalina Island Marine Institute, off the coast of California, where one of the fish was discovered.
“It was very cool to see, hold, and study an animal that we have never seen before. For me it was like being a kid on Christmas and opening up an amazing present.”
Marine scientists dissected the body and sent specimens to other research organisations interested in studying the gigantic fish.
Maybe it's a combination of causes
But no one has yet been able to prove why they come ashore. Theories about possible causes include storms, mating, starvation or illness.
In Japan there are even legends about oarfish being harbingers of earthquakes.
“The only one I would rule out is earthquake activity. If you plot the major earthquakes in California against the sightings there is no correlation,” says Mr Feeney.
“Other possible causes include red tide (algal bloom), predators, oceanic current changes, and boats. Maybe it's a combination of causes.”
It is hoped more frequent sightings and improved underwater research equipment could help scientists finally uncover the secrets of this once mythical animal.
You can follow BBC Earth on Facebook, Twitter and Instagram.