It's an ancient reflex. Whenever Earth's surface—always a dicey place—becomes even less safe, people go below it, hoping that the latest wave of insanity will wash harmlessly over their heads. Creating underground sanctuaries against human aggression dates back to prehistory and hit its modern zenith shortly after September 15, 1961, when Life magazine's cover story promised to reveal "How You Can Survive Fallout." Inside, a chilling letter from President Kennedy urged Americans to learn how to build fallout shelters. As cold war tensions ebbed in the 1990s, many domestic bunkers were demolished or converted to wine cellars. After the terrorist attacks of September 11, 2001, most Americans fought the burrowing reflex: They watched the news and worried. Some planned makeshift aboveground sanctuaries made of plastic sheeting and duct tape. A few embraced the old urge and sought to make safe havens below the sod. One New Hampshire company reported quadrupled sales for its shelters, including a $64,000 egg-shaped fiberglass module that is shipped fully outfitted and ready to drop into a hole. But the smallest minority of all smiled grimly and rubbed more petroleum jelly on the blast-door gaskets. These are the ones who built or bought large, serious shelters decades ago and kept them in fighting trim, convinced that hardened bunkers, air locks, and decontamination showers would never go out of style.