Out on the smooth sea ice north of Svalbard, a male polar bear walks with a slow, rolling stride. From far away, he blends into the white landscape, which helps him stay hidden. But the bear isn’t actually white. Each of his guard hairs is clear and hollow, scattering light, and his skin underneath is jet black to absorb the little sunlight that reaches the Arctic.

Scientists know him as Ursus maritimus, or sea bear, and he is the largest land carnivore alive today. Adult males usually weigh between 300 and 800 kilograms and measure 200 to 250 centimeters from nose to tail. Females are about half that size, weighing 150 to 300 kilograms. Males have broader heads and heavier shoulders than females. Compared to brown bears, polar bears have leaner bodies, longer necks, and flatter skulls, which help them swim and reach into seal breathing holes.

Every part of the polar bear shows its life in the sea. Under its fur, a thick layer of blubber up to 11 centimeters deep keeps it warm, along with a dense undercoat. Its paws are huge, often 30 centimeters wide, with hairy soles that grip the ice and spread its weight like snowshoes. The claws are short and curved, perfect for grabbing seal skin instead of digging for roots. Polar bears are considered marine mammals because they spend most of the year on the frozen ocean rather than on land.

Polar bears live around the North Pole in five countries: Canada, the United States (in Alaska), Greenland, Norway, and Russia. There are nineteen recognized groups that follow the changing sea ice. Some bears stay near the shore ice all winter, while others travel for thousands of kilometers on drifting ice from the Beaufort Sea toward Siberia. When summer melts the ice they need, many bears come ashore and survive mostly on their stored fat.

Seals make it possible for polar bears to wait out the summer. Ringed seals and bearded seals provide most of their food, usually caught by waiting quietly at breathing holes. A bear will find a hole, sit downwind, and stay still for an hour or more until a seal comes up for air. Then it strikes, pulls the seal onto the ice, and eats the blubber first, sometimes leaving the rest for arctic foxes and gulls. Walrus, beluga carcasses, and the occasional bird or egg help fill in, but seal blubber is the main energy source that allows females to nurse their cubs during long periods without food.

Polar bear reproduction follows the rhythm of the ice. Mating takes place in April and May, but the fertilized egg waits to implant until autumn. In October or November, a pregnant female digs a den in a snowdrift, usually on land but sometimes on stable ice. She gives birth to one or two blind cubs in December or January, each weighing less than a kilogram. The mother does not eat or drink for months inside the den, using her body fat to make milk. Families come out in March or April, and the cubs stay with their mother for about two and a half years, learning how to find breathing holes and read the ice.

It is difficult to count polar bears across their huge range, but the IUCN lists them as vulnerable, with about 22,000 to 31,000 worldwide. Some groups are doing better than others. The Southern Beaufort Sea population near Alaska and western Canada has dropped as ice-free seasons get longer. The main long-term threat is the loss of sea ice due to climate change, which gives bears less time to hunt and pushes them onto land, where they face more people, less food, and lower cub survival. Pollution and industry add more stress, and the United States lists polar bears as threatened under the Endangered Species Act.

In the wild, most polar bears live 15 to 18 years, though some females can reach their late twenties. In zoos, they can live past 30. Polar bears can swim for hours, have been tracked traveling over 100 kilometers without stopping, and can smell a seal from nearly a kilometer away. For Inuit communities, who have lived with polar bears for thousands of years, the bear is called nanook and is respected and featured in stories, hunting traditions, and place names.

As the light fades, watch the same male bear. He stops at a ridge in the ice, sniffs the air, and then lies down next to a dark hole. He will wait. For at least 120,000 years, the ice, the seal, and the bear have repeated this scene. Now, what is changing is how long the ice lasts each spring.