Mark Zuckerberg is said to have started work on Koolau Ranch, his sprawling 1,400-acre compound on the Hawaiian island of Kauai, as far back as 2014.
It is set to include a shelter, complete with its own energy and food supplies, though the carpenters and electricians working on the site were banned from talking about it by non-disclosure agreements, according to a report by Wired magazine. A six-foot wall blocked the project from view of a nearby road.
Asked last year if he was creating a doomsday bunker, the Facebook founder gave a flat "no". The underground space spanning some 5,000 square feet is, he explained, is "just like a little shelter, it's like a basement".
That hasn't stopped the speculation - likewise about his decision to buy 11 properties in the Crescent Park neighbourhood of Palo Alto in California, apparently adding a 7,000 square feet underground space beneath.
Though his building permits refer to basements, according to the New York Times, some of his neighbours call it a bunker. Or a billionaire's bat cave.
