The blue whale is the largest animal known to have ever lived, bigger than any dinosaur.

Adult blue whales typically measure 24 to 30 meters, with females slightly larger than males. Whalers in the early twentieth century recorded individuals near 30 meters, and the heaviest reliably weighed specimens topped 150 tonnes. That is heavier than about 25 African elephants.

Giant sauropod dinosaurs, such as Argentinosaurus, may have rivalled blue whales in length, but estimates of their weight generally fall below the heaviest whales. Water supports a whale's body, allowing a size that would be very difficult for an animal walking on land. A 2023 study suggested an ancient whale, Perucetus, might have been heavier, but other scientists have challenged that estimate, so the blue whale keeps its title for now.

Remarkably, this giant feeds almost entirely on krill, shrimp like animals only a few centimeters long.

Commercial whaling devastated blue whales in the twentieth century. Explosive harpoons and factory ships made it possible to hunt these fast swimmers, and hundreds of thousands were killed, especially in the Southern Ocean. By the 1960s their numbers had fallen dramatically, and the International Whaling Commission banned commercial hunting of blue whales in 1966.

Populations are slowly recovering, but blue whales are still listed as endangered by the IUCN. Today the main threats are collisions with ships, entanglement in fishing gear and ocean noise. Scientists study them with satellite tags, drones and underwater microphones, learning how they find dense swarms of krill and how far they travel between feeding and breeding areas.

On land, bones and muscles must hold up an animal's full weight against gravity. In water, buoyancy carries much of that load, letting whales grow larger.

Studies suggest whales became truly gigantic only in the last few million years, as ocean conditions concentrated food into rich seasonal hot spots.

Size has costs and benefits. Large bodies hold heat well in cold oceans and can store a lot of energy, which helps whales migrate long distances between feeding and breeding grounds. But staying big requires vast amounts of food, which is why blue whales depend on dense krill swarms.

Their feeding style, called lunge feeding, is one of the largest biomechanical events in the animal kingdom. A whale accelerates, opens its mouth and engulfs a volume of water that can be larger than its own body, then pushes the water out through baleen.

The Natural History Museum in London hangs a real blue whale skeleton, nicknamed Hope, in its main hall. It is more than 25 meters long and has become a symbol of the species' recovery and of ocean conservation.