Building on permafrost can be risky business

Permafrost

Permafrost is soil, silt and rock located in perpetually cold areas that remain at or below a temperature of 0° C for long periods. The minimum period is from one winter, through the following summer, and into the next winter; however, most permafrost has existed for much longer. Ground ice usually exists at temperatures close to its melting point and is liable to melt if the ground warms. A thin layer may thaw during summer months, though the majority of the permafrost in a given location will remain frozen until the local climate dramatically changes, as from continental drift toward the equator or global warming.