The bunker was erected quickly. Ideally, it would have been built deep underground, but as Nantucket is a relatively small island, digging down meant hitting water. Instead, the shelter was made up of Quonset huts; prefabricated structures that were developed during World War II.

Earth was then piled on top of the huts and scrub was left to grow over the shelter, camouflaging the site from any prying Soviet spies. While that doesn't sound like much, “a shelter only needs three feet (0.9m) of dirt on top, and you can stay there safely until the radiation levels have come down,” bunker-building expert Bradley Garrett told the Smithsonian Magazine.

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Inside the world's most fascinating secret bunkers