The deep sea is by far the largest habitat on Earth. It is, says Natalie Lawrence in her book Enchanted Creatures, “the last wilderness vast enough to hide monsters”.
Unsurprisingly, many myths have arisen about the terrifying things that lurk in these ink-black waters. But giant sea beasts are not just the stuff of legend, they actually exist.
- What the deep-sea robots saw when they went to the darkest depths of our oceans
- How deep is the deepest part of the ocean?
And now, thanks to the latest generation of remotely operated vehicles and underwater drones, we are gaining thrilling insights into this hitherto mysterious deep-sea megafauna. The colossal squid, which may weigh up to 500kg and has eyes the size of footballs, was filmed for the first time as recently as March 2025.
Supergiant amphipods have also been captured on camera. Relatives of garden woodlice, the monstrous scavenging crustaceans were discovered scuttling around in mid-ocean trenches 7km beneath the waves, and can grow to more than 30cm long.
In 2015, scientists found a sponge as big as a van at the bottom of the Pacific near New Zealand. The immense invertebrate was quite possibly thousands of years old.
Footage of enormous deep-sea sharks, octopuses and spider crabs has also gone viral in recent years. Why, though, are these creatures so much larger than their relatives at the surface of the ocean?
A leading theory for gigantism, as the phenomenon is known, focuses on food availability. Compared to the sunlit surface waters, the deep sea is effectively a desert, where food is both scarce and unpredictable. The animals that live here cannot afford to miss a single meal that comes their way and, when they get lucky, it makes sense to eat as much as possible.
In this environment, feast and famine is the name of the game. Larger animals are clearly at an advantage for the simple reason that they’re able to stuff themselves silly and can store more of their substantial energy intake as fat.
Huge 14-legged crustaceans called giant isopods are masters of the blow-out meal. Following a heavy feeding session, these seafloor scavengers can, incredibly, go without eating for four years. And, of course, larger animals can also travel greater distances to track down food. It all adds up to one thing – in the deep sea, big is definitely beautiful.
But there is probably another factor at play, something that turns tiddlers to titans – and that is water temperature. The water in the deepest parts of the ocean is much colder than at the surface. For example, in the abyssal zone (4,000–6,000m deep) it’s usually 0–4°C. This has a significant effect on the metabolism of everything found there. Cool conditions slow down an animal’s body processes, which, in turn, means it uses up precious fat reserves more slowly.
In other words, the energy needs of deep-sea creatures are pretty minimal. Consider the colossal squid we met earlier. This massive mollusc can get by on several fish a year. In calorific terms, that’s more or less an apple a day.
It just so happens that a consequence of living slowly and efficiently is a tendency to great age and size. Greenland sharks, which move so sluggishly it hardly counts as swimming, can live to the grand age of 500, making them the oldest living of all vertebrates. Growing at the rate of barely 1cm each year, they eventually become ocean giants.
Oarfish, whose bizarre red crests seem straight out of a horror film, are another example of deep-water giants that takes things extremely slowly. Their elongated, ribbon-shaped bodies reach a length of 8m – similar to the average garden hose – and doubtless inspired legends of gigantic sea serpents.
Who knows what other giants of the deep remain to be discovered?
Discover more amazing wildlife stories from around the world
- “It’s like swimming inside a snow globe.” When this diver dropped into a remote reef, dozens of huge animals appeared from the darkness
- Explorers drop 6 miles into Japan's deepest abyss, then 29,556 bizarre organisms emerge from the darkness
- Oldest living animal ever found suggests Atlantic Ocean is close to tipping point, say scientists
- Night divers capture rare images of fish carrying stinging anemones in their mouths
Top image: a greenland shark. Credit: Hemming1952, CC BY-SA 4.0, via Wikimedia Commons
