Figure 21.20Permafrost melting could destabilize structures at high latitudes, such as the Trans-Alaska oil pipeline, whose 1300-km (800-mile) route from Prudhoe Bay to Valdez crosses 675 km of permafrost. Where the pipeline crosses permafrost, it is perched on specially designed vertical supports. Because thawing of the permafrost would cause the supports to become unstable, they are outfitted with heat pumps designed to keep the ground around them frozen. The heat pumps contain anhydrous ammonia that vaporizes below ground and rises and condenses above ground, discharging heat through the two aluminum radiators atop each of the vertical supports.