Clip: Season 17 | 2m 53s
By September of 1942, the Army Corps of Engineers building the Alaska Highway had struck permafrost—ground that had been frozen for two or more years. After removing the covering vegetation, the frozen ground began to thaw, creating mud. Permafrost wasn’t the only challenge the engineers faced. In October, temperatures began to drop, signaling the start of one of the coldest winters on record.
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