We're already contemplating our retirement home. We'll be buying acreage and building our dream home in the form of an earth sheltered home: http://www.daviscaves.com http://www.formworksbuilding.com/ and we might put up a couple of small earth sheltered cabins on the property and rent them seasonally. ZERO maintenance. But when we do build OUR home, I have already determined that we're going to build a large walk in safe with a VAULT DOOR that can be opened from the inside just in case (LOL). The walls are all heavily rebar reinforced concrete and this should work really well. You can get stand alone vault doors from Fort Knox Safes among others. I'm a "secret area" nut so there will also be other secret rooms...one perhaps to house a bar for parties and a hidden back exit with tunnel to get out if things get too hot. Most of my secret stuff will not be on the plans filed with whomever the "controlling legal authority" happens to be.