Rare sightings of giant phantom jellyfish — deep-sea creatures that look like UFO spaceships with thick ribbons streaming from their undersides — have been reported by cruise liner passengers who spotted the otherworldly animals off the coast of Antarctica, a new study finds.

The giant phantom jellyfish (Stygiomedusa gigantea), one of the deep sea's largest invertebrate predators, met the guests while they were riding in a submersible deployed by cruise line operator Viking in early 2022. Researchers estimated that the jellyfish were longer than 16 feet (5 meters), with one stretching to at least 33 feet (10 m) in length, according to a study published Jan. 30 in the journal Polar Research.

Study first author Daniel Moore first realized guests had encountered the giant phantom when he saw a picture of one on a guest's camera. "I instantly recognised it for what it was and, given the rarity of sightings, was flooded with excitement," Moore, one of Viking's chief scientists, told Live Science in an email.

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Giant Phantom jellyfish live in every ocean except for the Arctic Ocean. However, because these cryptic creatures typically swim deep below the surface, they are scarcely seen by humans. The new study describes direct observations of three different jellyfish made during submersible dives off the Antarctic Peninsula.

"On every sighting the jellyfish appears to be swimming slowly, gently pulsing its bell for propulsion," Moore said. "They don't appear to have shown any inclination towards the lights of the submersible or reaction to our presence."

The jellyfish were spotted at depths of 260 feet (80 m), 285 feet (87 m) and 920 feet (280 m). Giant phantom jellyfish primarily occupy depths of below 3,280 feet (1,000 m), but they are encountered higher up in the Southern Ocean, or Antarctic Ocean. It's not yet known why they hang out in relatively shallow waters around Antarctica.

Moore noted that one potential explanation is that the jellyfish swim higher up to expose themselves to ultraviolet radiation, which will rid them of parasites. Another hypothesis put forward by Moore is that the upwelling deep water found around the Antarctic continent simply carries them upward. Moore hopes that their observations will lead to a better understanding of giant phantom jellyfishes' lives.

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The practice of cruise lines taking passengers to Antarctica has attracted some controversy. The U.S. Coast Guard announced on Feb. 2 that it has joined international partners to investigate four deaths and other casualties involving U.S. citizens on Antarctic passenger vessels between Nov. 15 and Dec. 1, 2022. This includes one death on the Viking Polaris, operated by Viking, after a large wave hit the ship.

The U.S. Coast Guard describes the Antarctic as a "unique high-risk" environment and aims to improve marine safety and prevent similar incidents in the future. The waters around Antarctica can be treacherous and the continent has a history claiming intrepid explorers in famous expeditions.