Mr. Hall has converted a former military nuclear missile vault into a luxury condominium built 15 stories into the Earth's crust. He is a leader among a new group of real estate developers investing in the nation's central prairies and Western foothills: doomsday capitalists.

Americans have, for generations, prepared themselves for society's collapse. They built fallout shelters during the Cold War and basement supply caches ahead of Y2K. But in recent years, personalized disaster prep has grown into a multimillion-dollar business, fueled by a seemingly endless stream of new and revamped threats, from climate change to terrorism, cyberattacks and civil unrest.

Bunker builders and brokers have emerged as key players in this field. And they see the interior of the country, with its wide-open spaces, as a prime place to build. Aiding them is history.