The oarfish, the longest bony fish species which can grow to more than 50 feet, is rarely seen - dead or alive - and is believed to be the origin of sea serpent legends.
Jasmine Santana, 26, of the Catalina Island Marine Institute (CIMI) was snorkeling with colleagues in Toyon Bay, southern California last week when she spotted something shimmering in the water.
Santana told 'New York Daily News' that she got nervous, and her heart started beating faster as she realised it was an oarfish because she had seen a rare video of a smaller one.
"About 15 more of the staff got down to lug it out and bring it on shore. I've been here for a little over 10 years, and I've not seen one at all," said Jeff Chace, a CIMI programme director.
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While the fish was dead, it was nearly completely intact and appeared to have died from natural causes.
Oarfish are thought to dive more than 3,000 feet down into the darker depths of the ocean, so their behaviour remains largely unobserved and unstudied.
However, far from the ferocious monsters of legend, the oarfish has a physiology that suggests it is relatively harmless, Santana said.
The oarfish is currently on ice, but Chace wants to bury it on the beach in the hope that other animals will eat away its remaining tissue so that CIMI can display its skeleton.