The crew of sailing school vessel Tole Mour and Catalina Island Marine Institute instructors with an oarfish that was found off Catalina Island, Calif., on Sunday. Catalina Island Marine Institute/AP

A California marine instructor's leisurely weekend snorkeling excursion turned into the discovery of a lifetime when she found the carcass of a massive, eel-like creature thought to have inspired legends of giant sea serpents.

Catalina Island Marine Institute (CIMI) instructor Jasmine Santana spotted the 18-foot oarfish, which is as thick as a man's torso, while snorkeling in clear waters off the island's coast on Sunday afternoon, the institute said.

She needed more than 15 helpers to drag the giant sea creature, with eyes the size of half dollars, to shore.

"We've never seen a fish this big," said Mark Waddington, senior captain of the Tole Mour, CIMI's sail training ship. "The last oarfish we saw was 3 feet long."

The creatures are found in all temperate to tropical waters, but because they dive to depths of 3,000 feet, they are rarely seen and remain largely unstudied. Little is known about their behavior, the nonprofit educational institute said.

The fish apparently died of natural causes. Tissue samples and video footage were sent to be studied by biologists at the University of California, Santa Barbara.

Santana spotted something shimmering about 30 feet deep while snorkeling during a staff trip in Toyon Bay at Catalina Island, about two dozen miles from the mainland.

"She said, 'I have to drag this thing out of here or nobody will believe me,'" Waddington said.

After she hauled the carcass by the tail for more than 75 feet, staffers waded in and helped her bring it to shore.

Oarfish have a pug face, a crest running the length of their bodies and a skeleton of bone rather than the cartilage common to fish species such as sharks. They can grow to a length of 56 feet, making them the longest bony fish in the world, according to CIMI.

Because of their strange appearance, they are believed to have inspired legends of giant sea serpents.

A CIMI spokesman said the sea creature's skeleton will likely be put on display for visitors.

Al Jazeera and wire services