Because they spend most of their lives on the sea ice of the Arctic Ocean depending on the ocean for their food and habitat, polar bears are the only bear species to be considered marine mammals.
Male polar bears can weigh up to 800kg, and are twice the size of females, they can measure up to 3 meters long making them the largest land carnivore in the world.
Polar bear fur appears to be white but actually their fur is translucent and reflects visible light making it appear white and beneath all that thick fur, their skin is jet black!
Polar bears can swim for long distances and up to speeds of 6mp to get from one piece of ice to another. Their large paws are specially adapted for swimming, which they’ll use to paddle through the water whilst using their hind legs flat like a rudder.
Climate change is the greatest threat to the polar bear’s survival, but there are other hazards including the oil and gas industry. Habitat destruction from oil exploration work and contact with oil spills which can reduce the insulating effect of a bear’s fur requiring them to use more energy to get warm, and can poison them if ingested are real dangers to the survival of this species. Through ingesting toxic chemicals such as pesticides through their prey, a bear's biological functioning and ability to reproduce can be affected.
Melting sea ice from climate change has increased human-polar bear conflicts when hungry polar bears go searching for food in the summer. Fortunately, people are learning to adapt to the polar bear's presence and take preventative measures to reduce the risk of conflict. 
Polar bears have a very strong sense of smell, which they use to find seal breathing holes in the ice. Once it has found the hole, the bear will wait patiently until the seal comes up for air to attack. They can even detect a seal in the water beneath a metre of compacted snow.