The colossal squid can grow to 23 feet and weigh half a ton in the ocean’s darkest depths

For a species that ranks among the largest invertebrates on the planet, the colossal squid has spent almost its entire scientific existence unseen. It was named a century ago from remains hauled up in the stomachs of whales and off the lines of deep-sea trawlers, a creature reconstructed largely from its own dismembered parts. Only recently has anyone watched one alive in its home, and even that glimpse was of a tiny juvenile, a reminder of how much of this animal’s life still unfolds beyond human reach in the cold black water of the Southern Ocean.

An animal built to superlatives

The colossal squid, Mesonychoteuthis hamiltoni, is not merely large but record-setting in specific, measurable ways. Adults are estimated to reach up to seven meters, or about 23 feet, in total length, and the heaviest confirmed specimens have weighed close to half a ton, which by mass makes the species the heaviest invertebrate known to science. According to a profile of the species, that combination of length and bulk sets it apart even from the more famous giant squid, which grows longer in the tentacles but far lighter overall.

Its eyes may be its single most extreme feature. Each can span roughly the width of a dinner plate, the largest eyes of any known animal, living or extinct, an adaptation for gathering the faintest traces of light in a realm where sunlight never penetrates. Those enormous eyes are thought to help the squid detect the bioluminescent glow given off by prey and by the sperm whales that hunt it. Its arms and tentacles carry rotating, three-pointed hooks rather than the simple suckers of many squid, a formidable arsenal for seizing prey in the dark.

A century known only in fragments

The species was formally identified in 1925 from two arm crowns recovered from a sperm whale’s stomach, and for decades that was roughly the state of the science: a picture assembled from pieces. Whales remained the most reliable source of specimens, since sperm whales dive deep enough to hunt colossal squid and sometimes surface bearing the scars of the encounter. Occasional intact animals came up dead or dying in fishing gear, most famously a large specimen landed in the Ross Sea, but a living colossal squid observed at depth stayed stubbornly out of reach.

That gap is why the animal became a kind of holy grail for deep-sea biologists. It is thought to live at depths of hundreds to a thousand meters or more around Antarctica, a habitat that is expensive, cold and technically punishing to explore. Cameras that might capture it must contend with crushing pressure, near-total darkness and the simple problem that a creature this rare could go unseen for years even when researchers are looking directly for it.

The first look at a living one

The long wait ended in 2025, a century after the species was named. On an expedition near the South Sandwich Islands in the South Atlantic, a remotely operated vehicle recorded a colossal squid alive in its natural habitat for the first time. The team behind the footage filmed the animal at roughly 600 meters down, capturing a delicate, semi-transparent creature drifting in the beam of the submersible’s lights.

The catch, fittingly for such an elusive species, was that the star of the footage was a baby. The individual measured only about 30 centimeters, nearly a foot long, a fraction of the size an adult can reach. As a summary of the discovery noted, dying adults had been seen before at the surface, but no one had ever documented a healthy colossal squid at depth, which made even a juvenile a landmark observation. Its slender body and glassy fins looked nothing like the monstrous predator of popular imagination, underscoring how little is known about how the animal changes as it grows toward its half-ton maximum.

Why the depths keep their secrets

The colossal squid endures as a case study in how much of the ocean remains genuinely unexplored. An animal that can outweigh a grand piano managed to elude direct observation for a hundred years, not because it is mythical but because its world is so hard to reach. The deep sea is the largest habitat on Earth by volume, and its inhabitants are adapted to conditions that make sustained human observation costly and rare.

Each new sighting sharpens questions rather than closing them. Researchers still do not know precisely how large the biggest adults grow, how long the animals live, how they hunt, or how their populations are faring as fishing and warming reshape the Southern Ocean. The single juvenile caught on film hints at answers while confirming how far the science still has to go. For now the colossal squid remains what it has always been, a giant defined by its absences, glimpsed only at the edges of a world humans are still learning to see.

This article was produced with the assistance of AI and reviewed by the Morning Overview editorial team.

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