Some of the world's richest tech moguls are not just investing in the next startup. They are investing in their own survival. Mark Zuckerberg is digging bunkers in Hawaii. Sam Altman owns gold, gas masks, and land to escape to. Reid Hoffman jokes that a house in New Zealand is now a high-status form of “apocalypse insurance.” They call it “doomsday prepping,” but this isn’t the average guy stocking canned beans in his garage. This is billionaire-level prepping, like private compounds, underground shelters, and escape...