Conjure in your mind a giant, deep-sea predator, and I bet there's a colossal squid lurking in there, perhaps with an even bigger sperm whale chasing after it.
But there are plenty of other enormous hunters prowling the dark depths that still remain great mysteries to science, including the aptly named giant phantom jellyfish, Stygiomedusa gigantea (from Greek words translating roughly as the “giant jellyfish of the underworld”).
These impressively proportioned jellies are generally a deep crimson colour, with a smooth, rounded bell (or ‘head’) a metre (3ft) or more wide, and fringed in a rippling skirt.
Four ribbony arms trail behind for up to 10m (33ft) – long enough that if one of these jellies was sitting in the back row of seats in a London double-decker bus, it would be able to tap the driver on the shoulder.
Giant phantom jellyfish are members of the Ulmaridae family, together with more familiar species such as moon jellies.
Unlike their close cousins, however, the phantom jellies don’t have stinging cells to hunt and paralyse their prey. Instead, they grab small fish and zooplankton in their great, long arms and shove them into their mouths.
Scientific explorers first dragged up a specimen of a giant phantom jellyfish at the end of the 19th century, and the species was officially named in 1910.
In the decades since then, and despite their size, there have been just over a hundred recorded encounters of phantom jellies.
Rare sightings have been made all around the world, in the Atlantic, Pacific, Indian and Southern Oceans, where they generally occupy extreme depths. They’re known to roam more than 6km (4 miles) down.
In 2025, scientists on the Schmidt Ocean Institute’s ship the R/V Falkor (too) were exploring Antarctic seas, where a massive iceberg had recently calved off an ice sheet, uncovering a huge swathe of the sea that, until then, had been out of researchers’ reach.
Using the remotely operated vehicle SuBastian, they discovered an ecosystem that had been thriving on the seabed under the ice, full of animals like octopuses and giant sea spiders.
They also obtained spectacular footage of a giant phantom jellyfish drifting past.
More of these mysterious jellyfish have been spotted by tourists than by scientists. High-end Antarctic cruise ships are equipped with personal deep-diving submersibles to take holidaymakers on excursions beneath the waves.
Several passengers have been lucky enough to encounter a giant phantom jellyfish and have returned to the surface with ghostly video footage on their mobile phones.
A scientific paper published in 2023 reported on these sightings, which took place mostly around the coasts of the West Antarctic Peninsula between 80–280m (262–918ft) underwater.
To put this in perspective, that’s roughly between the heights of Big Ben and the Empire State Building.
Why these giant phantom jellies range closer to the surface in the icy cold waters of the Southern Ocean is just another, as yet, unsolved mystery.
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