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Permafrost is perennially frozen soil that has been below zero degrees C for at least two years. It's found underneath about 25% of the northern hemisphere, mainly around the Arctic - but also in the Antarctic and Alpine regions. It can range in depth from one metre under the ground all the way down to 1,500m. In Alaska, the warming of the permafrost has been linked to trees toppling, roads buckling and the development of sinkholes. [Source: BBC News]
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