Apple, and more recently Samsung, is known for taking extreme measures to keep products secret while they’re being developed. According to Tech Radar, Microsoft employed similar techniques when developing its Surface tablet. The team behind the software giant’s latest foray into the hardware business started developing the slate in a secret underground bunker that contained no windows. As the team grew they moved into a larger building, one that included windows, but also included bank vault security measures such as airlock-style doors. Microsoft’s secrecy payed off, and the Redmond-based company even surprised some of its manufacturing partners with its announcement.