A Nevada family of scuba divers has prototyped an underwater fort - an anchored air pocket in a net - at the secret location at the bottom of a lake in the Sierras. And it's been there for three years!
It's made with vinyl, mounted under a net, mounted to an octagon of steel. I'm going to say that this is Lake Tahoe, but there are certainly many large lakes in the Sierras, so I could be very wrong. Interview with the family over at BoingBoing.