Soldiers and civilians working on the highway faced extreme challenges including temperatures that would dip to -60 degrees F in the winter. Permafrost (sections of land that are permanently frozen and buried by insulating moss called muskeg) drove the engineer’s bananas. When you strip the vegetation off, the otherwise very stable ground turns to mush. Eventually they built much of the highway right on top of the moss. However, it's still melting permafrost that causes the dizzying dips on the highway.