Winter in the Arctic and Antarctica mean temperatures as low as -70 degrees centigrade.
Saltwater freezes at -2 degrees centigrade, so during this time the Arctic Ocean is transformed, covered in a layer of ice 3m thick.
Winter ice covers 16 million km²
In Antarctica, winter ice advances across the Southern Ocean at an astonishing rate of 100,000 km² a day.
Winter ice covers 20 million km²
These frozen oceans may seem similar, but they support very different ecosystems, above and below the frozen surface.
In the Arctic, polar bear patrol the sea ice, hunting seals and beluga.
Seals have developed different survival strategies for their pups:
Hooded seals breed on unstable ice where polar bears will not venture.
Harp seal pups are white for camouflage.
And ringed seals hide their pups beneath the snow.
In Antarctica, there are no land predators. Emperor penguins can spend the winter on the ice in relative safety.
Their main predator waits for them to enter open water - the leopard seal.
In spring, both frozen oceans begin to melt.
The Arctic sea ice shrinks from 16 to 9 million km².
Belugas move to shallow waters to rub their molting skin away.
Walrus feed on clams, guillemots dive up to 50m for fish, and mysterious narwhal follow channels in the sea ice to fishing grounds.
In the Southern Ocean, enormous icebergs are released into the sea.
And krill swarm in their billions, attracting fur seals, minke whales, and humpback whales, which can eat two tons of krill a day.
For two frozen, seemingly inhospitable environments, there is an abundance of life to be found in the Earth's frozen seas.