Greenland by dog sledge: The Sirius Patrol in numbers
Since the Cold War, Denmark has staked its claim to northern Greenland - and its untapped mineral wealth - with dog sledge patrols. This is the Sirius Patrol in numbers.
The vast icy expanses at the top of our planet are rich in coal, ore and minerals. Although inhospitable in the extreme, these areas are in demand.
Denmark patrols and protects its sovereignty over Greenland with a small naval unit called the Sirius Patrol. The US Geological Survey estimates the oil reserves off its coast are as big as those in the North Sea.
Each autumn, six dog sledge teams - each manned by two soldiers from the Royal Danish Navy - spend up to six months patrolling an area of 160,000 sq km (60,000 sq miles). They are the only people in a vast wilderness about three times the size of Denmark.
During winter the sun disappears for two months. The average yearly temperature is -10C (14F) and the mercury can dip as low as -55C (-67F) - the lowest recorded temperature in the area.
There are up to 14 dogs in each team, and a day's patrol will typically cover 30km (19 miles). At night the soldiers retire to a hi-tech tent. The dogs sleep outside, even in the depths of winter.
The unit selectively breeds Greenlandic dogs for endurance and strength. Each dog works for five years. By the time it retires, a dog in the Sirius Patrol will have pulled sledges for more than 20,000km (12,427 miles).
During a two-year placement with the unit, the soldiers are paid a monthly salary of 22,000 Danish kroner (£2,535) after tax. Their arctic training includes dog handling, building emergency snow shelters, and hunting for food.
After being granted sovereignty over Greenland in 1933, Denmark has been obliged to maintain a permanent presence in the entire area.
The first dog sledge patrols began during World War II to monitor and then destroy German weather bases as part of efforts to keep Greenland in Allied hands.
In 1950, with the Cold War cooling international relations, Denmark decided to establish a permanent military presence. Initially christened Operation Resolut, it was renamed Sirius in 1953 after the brightest star in the dog constellation.
The Cold War has long since ended, but Greenland remains a desirable territory, rich in oil and precious metals. The environment is too extreme for current mining technology, but the patrols secure Denmark's claim to this valuable wilderness simply by their presence.
Frozen Planet is broadcast on BBC One on Wednesday 30 November at 2100 GMT, or catch up on iPlayer.