"When the permafrost melts, the house sinks with it. Building on permafrost is dangerous, since the heat of the building might just be enough to thaw the ground, causing serious damage. But builders in the Arctic have avoided this. Oil pipelines, water supply pipes, and houses are often elevated using stilts and carefully insulated so that they can stay warm without melting the frozen ground below. As well, roads and buildings are sometimes built on thick gravel pads, and in some cases heat extractors are buried deep in the ground to keep the permafrost cool."