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Polar bears thrive at this time of year thanks to the huge platform of sea ice on which they can hunt.
北极熊在这个时节活力旺盛，是受它们能够在其上狩猎的巨大海冰平台所赐。
The arctic ocean lies across the top of the northern hemisphere and over the north pole itself.
北极洋占据北半球的顶端，并且横跨北极。
And is surrounded by three continents -- Europe, North America and Asia.
也被三个大陆环绕--欧洲、北美洲及亚洲。
In winter, the sea ice extends across an area of over five million square miles.
在冬天，海冰遍布超过五百万平方英里的地区。
But off the coast of arctic Russia, a tiny pocket of water has been kept open.
但在北极俄罗斯的海岸，一个微小水孔持续开着。
By the continuous stirring of 30 trapped beluga whales.
因为30只受困白鲸不停搅动。
These four-metre-long arctic whales are superbly adapted for hunting far beneath the surface of the sea ice.
这些四米长的北极鲸绝佳地适应了海冰表面下深处的狩猎行动。
Nonetheless, being mammals, they must return to the surface to breathe every 15 minutes or so.
尽管如此，因为身为哺乳类，它们每15分钟左右就得回到表面呼吸。
But this cold spell is exceptionally severe.
但这个寒流特别严酷。
And there's not another breathing hole within 20 miles.
方圆二十英里内，没有别的洞得以呼吸。
Nearby fish and squid disappeared long ago.
附近的鱼与乌贼早就消失无踪。
These emaciated whales have been imprisoned by the ice for five months, and they're now starving.
这些瘦弱的鲸已经被冰囚禁了五个月，它们现在饿到不行。
Their only chance of escape will come when there is a change in the season.
唯一逃脱的机会将在季节转换之际来临。
As spring finally reaches the north of the planet, air temperatures creep above freezing.
当春天终于降临北半球时，气温悄悄上升到冰冻温度之上。
The impenetrable ice ceiling begins to fracture.
坚不可摧的冰层开始断裂。
Small life-saving channels open up, bringing freedom.
狭小却能救命的渠道打开，带来自由。
But spring is a dangerous time for beluga.
但春天对于白鲸来说是危险时分。
As tides and currents pull ice apart, the beluga find themselves in a labyrinth of shifting ice floes
当潮浪与水流把冰拉开，白鲸会处于由移动浮冰形成的迷宫之中，
that could close their breathing holes at any point, and so drown them.
可能随时会盖住它们的呼吸洞口，导致它们溺毙。
Using their sophisticated sonar, they try to find a way through the icy maze.
靠着它们精密的声纳，它们试图在浮冰迷宫之中找到出路。
And eventually reach a patch of open water.
最后抵达一大片开放水域。
They're not the only whales here.
它们不是该处唯一的鲸。
Narwhal, the unicorns of the sea, are closely related to beluga.
独角鲸，海中的独角兽，与白鲸血缘相近。
In a rarely seen encounter, these two kinds of whales appear to socialise.
在罕见的接触中，这两种鲸看起来像在社交。
Who knows what's going on between them?
天知道它们之间发生了什么事。
But soon, they set off together through the shifting fields of ice.
但很快地，它们一起出发，穿过移动的浮冰。
A journey of up to 1,000 miles. Before they reach ice-free waters, where they can rear their young.
一趟长达一千英里的旅程。直到它们抵达无冰的水域，好养育后代。
