Permafrost is a layer of soil or earth which is permanently below the freezing point. The geological definition does not require that water ice actually be present in order for a region to be defined as permafrost, though with some exceptions (like at bedrock layers), normally permafrost consists of rock or soil saturated with ice. The permanent or continuous permafrost which is ubiquitous in the Arctic (e.g. parts of Alaska, Siberia, and the Canadian northern territories) is bordered to the south by a region of so-called sporadic permafrost, which covers only part of the landscape and would disappear first after uncharacteristically warm periods.
The deep permafrost which has accumulated in Arctic North America is the result of hundreds of thousands of years of accumulation. One year can freeze only the top few metres of ground at most; to accumulate hundreds of metres of permafrost, as has occurred in some areas, must have involved the better part of a million years of growth, gradually stretching further downwards. Permafrost often leads to characteristic surface formations called patterned ground, taking the form of polygon-like wedges, squares, and rings as a result of soil upheaval. It was these patterned ground formations which NASA's Phoenix spacecraft seems to have identified in the Martian polar regions, and which was used to support the hypothesis that there may be underground water on some areas of Mars.
Global climate change, regardless of whether or not it is largely attributable to human industrial activities, may significantly alter the Arctic permafrost. The actual permafrost in the Yukon Territory, Canada, has been slowly receding for over a century as a result of very gradual natural warming cycles. It is feared that accelerated thawing as a result of global warming would release large amounts of trapped methane into the atmosphere. in turn accelerating the rate of global warming even further. During the last Ice Age, permafrost once stretched throughout Europe, much of China, Canada, and the northern continental American states.
Constructing permanent buildings on permafrost raises special engineering challenges, and really has only been attempted since the twentieth century. Any heated building will gradually warm and thaw the permafrost underneath it - a problem not only from an environmental perspective, but from the more immediate perspective that the building will slowly sink into the resulting swampy earth. The traditional method of avoiding construction problems is to build only on platforms suspended above the permafrost by wooden piles, although it is also possible to use heat pipes. Pipelines traversing the north face similar problems, and must be properly insulated to avoid warming their surroundings and causing sinking.