A mother polar bear prepares her cubs for a long journey, to avoid becoming stranded on floating glaciers. Including a behind-the-scenes insight into the filming of the documentary.
David Attenborough concludes the documentary series by focusing on the world's deserts. Cameras explore the dry Atacama in Chile, where guanacos survive by licking dew off cacti, and capture rare wild camels eating snow to stay hydrated on Mongolia's Gobi plains. In the rocky Namibian desert, elephants desperately search for food and lions wander the wilderness hunting oryx.
The team travel to the Ngogo Forest in Uganda, where they study the largest chimpanzee group in the world defending their territory against neighbours. Life-forms such as the flying colugo, red crab spiders, blue birds of paradise and the parasitic fungi cordyceps, which have developed unique survival abilities, are also observed
The Taiga forest, on the edge of the Arctic, is a silent world of stunted conifers. The trees may be small but filming from the air reveals its true scale. A third of all trees on Earth grow here and during the short summer they produce enough oxygen to change the atmosphere.
In California General Sherman, a giant sequoia, is the largest living thing on the planet, ten times the size of a blue whale. The oldest organisms alive are bristlecone pines. At more than 4,000 years old they pre-date the pyramids. But the baobab forests of Madagascar are perhaps the strangest of all.
Life goes to extraordinary lengths to survive this immense realm. A 30 tonne whale shark gorges on a school of fish and the unique overhead heli-gimbal camera reveals common dolphins rocketing at more than 30km an hour.
Descending into the abyss, deep sea octopus fly with wings and vampire squid use bioluminescence to create an extraordinary colour display. The first ever time-lapse footage taken from 2,000m down captures eels, crabs and giant isopods eating a carcass, completely consuming it within three hours.