Billionaires such as Mark Zuckerberg and Jeff Bezos are investing millions in building underground bunkers as a possible refuge in the event of an apocalypse.

Zuckerberg has spent $187 million on 1,600 acres in Hawaii, where he's building a luxury ranch that includes a 5,000-square-foot underground bunker. Podcaster Christina Randall says this is just one of about 15 such bunkers built by billionaires.