If you've ever taken Lincoln Boulevard through the Presidio here, you almost certainly didn't know that you passed within feet of one of the best-preserved World War II-era anti-aircraft machine gun nests in the country.

In fact, all around the Presidio are dozens of these original trenches and fox holes, most of which are completely grown over with weeds and other vegetation, but many of which still have the pillars on which Army crews once mounted their .50 caliber guns in preparation for an aerial or sea-based attack that, thankfully, never came.

As part of my Road Trip at Home series, I recently went on a tour with Stephen Haller, a National Park Service historian, of some of these sites, as well as many other of the hundreds of coastal fortifications built over more than a century from the Civil War to the Cold War that are located in and around San Francisco. And while some of them were built to house thunderous guns capable of firing on enemy ships as far as 25 miles off the coast, or to shoot nuclear-tipped missiles at invading bombers as high as 70,000 off the ground, Haller seemed more excited about these humble, weed-infested machine gun nests than anything else....