Watch the Birth of a Polar Bear | BBC Studios

The battlefield's enormous. Its size makes the polar bear a perpetual nomad, trekking 15 miles a day and up to 5,000 a year on ice that's constantly moving. Our story starts down there in the ice and snow. It's spring and a new life is about to see daylight for the first time. But not until this female tests the air. She's been holed up for nearly six months and she doesn't know what's out there now. There could be danger and she has something precious to protect. When she went into the den last winter, she was 50% heavier and alone. But she's not alone now. She has a cub, one cub. It's his first experience of the wider world. The first moments away from the warm, dark safety of the den. He was born in January, just over three months ago. The world he came into was small and cramped. He had fur, but his eyes were closed. He wasn't any bigger than a guinea pig.