In late December, researchers were descending a remotely operated vehicle in the Atlantic Ocean off Argentina when, at a depth of roughly 800 feet, the control room fell silent. On the screens, a jellyfish appeared. But it was no ordinary sighting: It was a giant phantom jelly, a deep-sea species rarely seen in the world’s seas.

“There was a mixture of excitement and disbelief,” said María Emilia Bravo, a marine biologist at the University of Buenos Aires, who led the dive as part of an expedition aboard R/V Falkor (too), a research ship operated by the Schmidt Ocean Institute. The institute released its findings on Tuesday.

“Its ethereal and delicate presence in such an extreme environment was deeply surprising,” she said. The creature’s long arms made it difficult for the underwater vehicle’s operators on the ship to maneuver safely. “We were curious about getting to know it better and documenting it well.”

First collected in 1899, giant phantom jellies were not recognized as a species until 60 years later. Even then, specimens were typically found dead in trawling nets and rarely documented alive. Remote-operated vehicles made sightings of these creatures possible.

“Up until recently, nobody was able to actually see it in its natural habitat and in its full glory,” said Steve Haddock, a marine biologist at the Monterey Bay Aquarium Research Institute who was not involved with the dive.

During the last century, phantom jellies have been documented about 100 times.

These elusive creatures are among the largest jellyfish: Their bells can grow over three feet in diameter, and their arms can reach up to 30 feet in length. People often assume that undescribed deep-sea species are obscure, hidden animals, Dr. Haddock said. “But no,” he said, “it’s these giant things, the size of a giant squid practically, that have gone largely unnoticed.”