The deep sea is a place of extremes — crushing pressure, freezing temperatures, and complete darkness. Yet it’s also home to some of the most mind-bending creatures on the planet. While marine science has made huge strides over the past few decades, there are still corners of the ocean we haven’t explored, and species we’ve only just discovered. What’s even more fascinating is that some of these creatures defy easy explanation. They don’t behave like anything we know, and in some cases, we’re not entirely sure what they even are. They challenge our understanding of biology, evolution, and survival in ways that are as humbling as they are awe-inspiring. Here are some of the most mysterious deep sea animals that continue to puzzle researchers to this day.
The Bloody-Belly Comb Jelly
This deep-red jelly looks like something out of science fiction. Found at depths of up to 5,000 feet, the bloody-belly comb jelly (Lampocteis cruentiventer) emits a dim bioluminescent glow, but thanks to its dark red colouring, it appears nearly invisible in the deep. Scientists believe the colour helps it hide from predators, as red light doesn’t travel far underwater. Its shimmering cilia — the hair-like paddles that propel it through the water — light up like a miniature rainbow. Despite being discovered decades ago, its feeding habits, reproduction strategy, and full range are still not fully understood. How it avoids predators, how it responds to environmental change, and its role in the food web remain open questions.
The Goblin Shark
With its flattened snout, nail-like teeth and protrusible jaws, the goblin shark looks more like a horror movie creation than a real animal. It’s been nicknamed the “living fossil” because its lineage goes back around 125 million years. Found in deep waters around Japan and the Atlantic, it can snap its jaw forward at lightning speed to catch prey — a behaviour scientists only recently captured on camera. Its soft, flabby body suggests it doesn’t need to move quickly or often, and researchers believe it likely ambushes slow-moving prey. Yet despite its ancient roots and terrifying appearance, we still know very little about its mating behaviour, gestation period, or even how long it lives.
The Giant Squid
For centuries, tales of giant sea monsters may have been inspired by sightings of the giant squid. It wasn’t until 2004 that one was even photographed in its natural habitat. These elusive creatures can grow to over 12 metres in length, with eyes the size of dinner plates — the largest eyes of any known animal. While we’ve found their beaks in the stomachs of sperm whales, actual live encounters remain extremely rare. Much about them remains mysterious: how they reproduce, how long they live, and how they communicate or navigate in the vast, dark waters of the deep ocean. Even the number of species within the genus Architeuthis is still debated.
The Barreleye Fish
Perhaps one of the strangest-looking fish ever caught on camera, the barreleye has a transparent, fluid-filled head that allows its tubular eyes to swivel inside its skull. These eyes are incredibly sensitive to light and are thought to help the fish detect the faint silhouettes of prey above. Found at depths of 600–800 metres, this fish was first discovered in the 1930s, but its unique anatomy wasn’t observed until 2009, thanks to deep-sea ROV footage. Researchers are still unsure how it feeds — some believe it steals food from siphonophores (long, jellyfish-like organisms), while others think it uses its precise vision to hunt actively. The reason for its transparent head, and whether it serves a defensive or offensive function, is still a mystery.
The Deep Sea Hatchetfish
Named for the thin, metallic shape of its body, the deep sea hatchetfish is a master of camouflage. It uses a process called counter-illumination — glowing just enough on its underside to blend in with the faint light from above. This makes it nearly invisible to predators looking up from below. It’s also able to detect and evade threats using highly sensitive photoreceptors that line its body. However, much about its life cycle remains a mystery. How often do they reproduce? What do their young look like? Do they communicate using light patterns? The answers are elusive, partly because observing them in their natural habitat is so challenging.
The Frilled Shark
The frilled shark is another so-called “living fossil” with a body design that hasn’t changed much since the time of the dinosaurs. Its eel-like shape and rows of backward-facing teeth give it an eerie, prehistoric look. It’s been found as deep as 1,500 metres, but sightings are extremely rare. It’s believed to hunt like a snake, lunging and swallowing prey whole. Some captured specimens have revealed unusual reproductive traits — including internal fertilisation and very long gestation periods that could stretch over three years. That would make it one of the longest-known pregnancies in the animal kingdom. Still, so much remains speculative due to the lack of live observation.
The Deep Sea Jellyfish (Stygiomedusa gigantea)
With a bell measuring over a metre across and trailing arms that can reach six metres, this jellyfish has only been seen around a hundred times since its discovery in 1910. Unlike many jellyfish, it doesn’t have the typical long, stinging tentacles. Instead, it uses thick, muscular oral arms to trap prey — a feature that baffles researchers. What kind of prey does it catch this way? How does it digest food? What predators, if any, does it face? These questions remain largely unanswered. The Monterey Bay Aquarium Research Institute has captured rare footage of this jellyfish gliding through the dark, but its role in the deep-sea ecosystem remains an open mystery.
The Faceless Cusk-Eel
Discovered again in 2017 by Australian researchers after more than a century without confirmed sightings, this ghostly white fish appears to have no face — no visible eyes or nose, and a mouth located underneath. Its strange appearance and elusive behaviour have made it one of the more haunting species in deep-sea biology. Scientists believe its sensory organs are highly adapted for the dark, but exactly how it finds food or mates is unknown. Its physiology challenges traditional ideas of how deep-sea fish function, raising further questions about what else might be living in the abyss beyond our reach.
The Dumbo Octopus
Named for its ear-like fins that resemble the Disney elephant’s ears, the dumbo octopus is found at extreme depths — sometimes more than 7,000 metres below the surface. It’s one of the deepest-living octopus species known to science. It propels itself by flapping its fins and steering with its arms, in a graceful motion that looks more like floating than swimming. But despite its endearing appearance, the dumbo octopus remains something of a mystery. We don’t know how many species there are in the genus, how they reproduce, or what their lifespan is. Capturing footage in their natural environment is difficult, so much of our knowledge comes from rare encounters with remotely operated vehicles.
The Zombie Worm (Osedax)
First discovered in 2002, these bizarre creatures earned their nickname from their habit of feeding on the bones of dead whales and other marine animals that sink to the seafloor. What’s most puzzling is how they do it — zombie worms lack mouths and stomachs. Instead, they rely on symbiotic bacteria to break down bone and extract nutrients. They also exhibit extreme sexual dimorphism: the females are visible and active, while the males are microscopic and live inside the females. How these unusual creatures evolved, how widespread they are, and how long their reproductive cycle lasts are all still being studied.
The Long-Armed Squid (Magnapinna)
With elbow-like joints and arms that can stretch over 7 metres long, the Magnapinna squid looks almost alien. Very few have been observed alive — most sightings come from deep-sea submersible footage, often near oil rigs or remote research sites. Their arms hang down in a bent posture unlike any other squid, and scientists aren’t sure why. Are they using their arms to detect prey, or to camouflage themselves? Their movement, anatomy, and even taxonomy are still being worked out. Because they live so deep — often more than 4,000 metres down — they remain largely out of reach.
The deep sea remains the least explored part of our planet. More people have been to the Moon than to the depths of the Mariana Trench. For every creature we’ve managed to film or capture, there are likely dozens — if not hundreds — that remain unseen. As technology advances, we’re finally getting glimpses into this alien world, but for now, many of its residents remain mysteries wrapped in darkness, silently drifting in a realm we’re only just beginning to understand. Every time we send a probe or ROV down, we find something new — and with each discovery, our sense of wonder deepens.