This 16-inch gun, which was installed during World War II across the Golden Gate Bridge at Battery Townsley, was one of the largest and most potent examples of coastal fortifications that the U.S. military installed in the area over more than a century.

SAN FRANCISCO--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.

"This is the best preserved World War II landscape in the lower 48 states," Haller told me. He explained that because the gun nests--one of which offered its crew a straight-on field of view of Baker Beach, and others of which covered crucial roads in and around the Presidio--were dug into such rocky substrate, they have remained in good shape all these decades. Similar positions on the East Coast would almost certainly have disappeared years ago, lost to eroding sands.

Even here, the gun nests were only found because Haller and others with whom he works here were looking at aerial photographs from World War II and saw a group of dugout positions they didn't recognize. After determining their location, they tracked some of them down and sure enough, they were still there, some in plain view of heavily-trafficked public roads.

This is one of the hidden World War II-era machine gun nests that can still be found in the Presidio of San Francisco.

But Haller said he and others at the National Park Service, which today operates the former military base known as the Presidio, as well as the entire Golden Gate National Recreation Area that spans San Francisco and the Marin Headlands just north of the city, haven't yet decided what to do about the positions. On the one hand, they recognize the historical value of the gun nests, and on the other, they worry about the damage that could be caused by thousands of people visiting what is fairly delicate territory. In the end, Haller explained, they will likely clean a small number of them up and make them available for public visiting, and keep mum about the rest.

America's most important Pacific port Today, there's a perception that San Francisco is home to a peace-loving population not too hot on the military. Whether that's true or not, that idea ignores the fact that San Francisco was once considered America's most important Pacific coast port. "In the 1890s, the Army began a major modernization of the Nation's coastal fortifications," a National Park Service document on the city's coastal defenses reads, "and, because of its strategic importance [due to burgeoning industry and easy access to inland areas of California], San Francisco Bay was given number two priority behind New York Harbor."

The history of defending the San Francisco Bay actually goes back well before the Civil War. The oldest location still remaining in the city--La Batteria San Jose, which boasted five iron cannons--was armed in 1797. A second wave of fortifications was begun in 1853, and San Francisco's famous Fort Point, which featured mounts for 126 cannons, was armed in 1861.

Indeed, a pamphlet entitled "Seacoast Fortifications of the Golden Gate" lists seven distinct eras of coastal defenses, starting with the Spanish-Mexican era and going all the way through the Cold War. Without a doubt though, the biggest remaining collection of batteries and other positions are from the Endicott period--which began around 1885--and the World War I and World War II periods. There was, as Haller put it, an unbroken line from the Civil War era facilities to the Nike Missile sites of the Cold War era.

Even as a lifetime resident of the San Francisco area, I was a bit stunned to look at this pamphlet and see that the city and the Marin Headlands just to the north comprised no less than 11 different forts and other military installations over the years.

As Haller put it, coastal defense has been a military standard that nations the world over have employed for hundreds of years in a bid to protect themselves from invasion. And the United States was no different. The many batteries and other installations around the area here are simply the modern manifestation of this.

And today, there is no shortage of sites that still remain in functional shape, though none have been operational since the 1970s and most were shut down by the late 1940s.

But big gun batteries take a very long time and a lot of resources to build, and they were expected to last for a generation or more. In most cases, the ones built in and near San Francisco did.

And because of their historical value, dozens of these sites are maintained by the National Park Service and are open to the public. Most have been stripped of any weaponry, and many are essentially shells of what were once very busy batteries. But at several sites, there are still relics of the weaponry that once was installed there, even if it's just for show.

Battery Chamberlin While it's easy to point to the Japanese as the enemy that might have tried to attack the Pacific coast during World War II, and the Soviets as being the likely attackers during the Cold War, one could fairly ask who the earlier coastal defenses were built to protect against.

Haller explained that the Civil War-era installations, like Fort Point, were probably in anticipation of some sort of attack by not-yet friends, the British. As well, he said, the French might have come our way with guns drawn.

By the early 1900s, though, the Japanese were already starting to come on strong, and were now seen as a potential attacker, Haller said. And that led to the creation of some of the biggest of all the batteries to be found in or near San Francisco.

One of the best stops anyone interested in coastal fortifications could make would be Battery Chamberlin, adjacent to Baker Beach in San Francisco. Originally armed in 1904 with four 6-inch guns, it remained operational until 1917, when the Army decided it needed most of its resources in Europe for what became known later as World War I.

"Built to protect underwater minefields laid outside the Golden Gate, this Endicott-era battery [had] guns mounted on disappearing carriages," a National Park Service Web page reads. "These guns had a range of 9 miles and could fire at the rate of 2 rounds per minute."

The disappearing gun (see video below) was an Endicott-era invention designed both to protect the guns, and to keep them hidden as much of the time as possible. "The disappearing carriage is a unique characteristic of Endicott-period fortifications," reads a placard at Battery Chamberlin. "Many larger caliber guns used this carriage. First invented in Europe, it was perfected in America with the addition of a counterweight system. A pair of massive steel arms connect the barrel and a lead counterweight.

"In loading or 'service' position, the gun is held down behind a protective parapet and the counterweight is suspended above a well. When the gun is released, the counterweight falls into the well, swinging the gun up into firing position. The recoil from firing them pushes the gun back into service position."

In 1920, Battery Chamberlin was re-outfitted with two six-inch guns, and during World War II, the Sixth Coast Artillery Regiment crewed the weapons, which were hidden beneath camouflage netting on the theory that the coast might be attacked by Japanese airplanes. Battery Chamberlin was finally decommissioned in 1948, as was the Coast Artillery Corps.

Today, the site is outfitted with a single six-inch gun. Though it was not originally used at Battery Chamberlin, and was installed in 1977 when the site was opened to the public as a museum, it remains a demonstration to the public of how the disappearing gun worked and what exactly a major military installation in the midst of a big city such as San Francisco was like. The museum is now regularly open to the public and the park service puts on demonstrations of the gun the first weekend of each month.

Another great site to visit is Battery Townsley, across the Golden Gate, in the Marin Headlands. Although it no longer contains weapons, the site "was a casemated battery that mounted two 16-inch guns, each capable of shooting a 2,100 pound, armor-piercing projectile 25 miles out to sea," a National Park Service Web page reads. "The guns and their associated ammunition magazines, power rooms, and crew quarters were covered by dozens of feet of concrete and earth to protect them from air and naval attack.

"This battery, named in honor of Maj. Gen. Clarence Townsley, a general officer in World War I, was considered the zenith of military technology and was the result of careful, long-term planning. As early as 1915, the Army was eager to construct the 16-inch gun batteries at San Francisco, and by 1928, the decision had been made to install two batteries near the city, one on either side of the Golden Gate straits. By 1940, Battery Townsley was completed and its two guns installed.

"Battery Townsley was a high security operation' civilians living in San Francisco knew that there were batteries nearby but their exact locations were not revealed. A battery of this design had never been actually fired before, so the soldiers underwent several months of practice before firing the guns for the first time."

Nearby as well, and jumping forward a few years, is SF-88L, a perfectly-preserved Nike Missile launcher site. Today, it is the only remaining site like that in the United States and it is often open to the public for viewings and demonstrations.

Nisei soldiers One of the most interesting yet unexpected things that the National Park Service's Haller showed me during our tour was a totally nondescript building near Crissy Field, on the northeast end of the Presidio. If you didn't know what it was, you'd never take a second look at the building. But in fact, it has a fascinating history.

"In 1941, when the United States faced the looming prospect of war with Japan, the War Department moved to develop linguists by directing the Fourth Army to open an intelligence school at the Presidio of San Francisco," reads a U.S. Central Intelligence Agency paper entitled "Nisei Linguists: Japanese Americans in the Military Intelligence Service during World War II." The school remained in San Francisco for just a year, when it moved to Camp Savage, Minn. But during the time it was resident in the Presidio, about 6,000 second-generation Japanese Americans--known as Nisei--were trained in the Japanese language in hopes that they could help with the war effort.

Indeed, the Nisei "contributed greatly to the U.S. victory over the Japanese...in the second World War and to the lasting bilateral alliance that followed," the CIA document reads.

Today, there is almost no recognition of the efforts of those 6,000 Japanese-Americans, but given that about 100,000 Japanese were held captive during World War II, the fact that so many Nisei actually assisted the American military during that time is notable. If you happen to stop at the building where the school was located, you might see an engraved sign outside that reads, "This building was the site of the first class of the Military Intelligence Service language school (from) November 1941 to April 1942. Eventually, six thousand Japanese-American soldier graduates served their country valiantly in the Pacific area during World War II. The United States of America owes a debt to these Nisei linguists and their families which it can never repay."

Sandbags in the hills One of the best things about having the Golden Gate National Recreation Area right in San Francisco's backyard, what with the care that the National Park Service has put into maintaining the coastal fortifications, is that if you put in enough time, or get lucky on a hike, you might stumble across something with some real historical significance.

This machine gun nest was used to protect Battery Townsley from attack.

To be sure, the large installations like Battery Townsley and several other very large batteries from the Endicott period to World War II-era facilities are visible to anyone and everyone. But sometimes, just around a bend, and over a small hill, there are things that are perhaps even more interesting, particularly because they've been left behind and for the most part ignored for decades.

For example, on the hill above Battery Townsley, there is an extremely well-preserved anti-aircraft machine gun nest, complete with a circular section of sandbags. While it, like the hidden gun nests in the Presidio, is grown over to some extent, this is much more visible. And you can see why the military put it there. It offers a terrific line of sight to most of the hillsides of the headlands, and it allowed the Army to protect Battery Townsley from above.

Close by, there's another relic--this time what appears at first to be a large drainage tunnel. In fact, though, it is was a Korean War-era bomb shelter. These types of things are scattered all over the Golden Gate National Recreation Area. And while most of them are not publicized for fear of their being destroyed by too many visitors, neither are there any impediments to checking them out--save for the fact that few know where they are. And while I won't reveal their location, I can say that they're right there, just up the hill from Battery Townsley. If you think like a military strategist would have, and consider what they were trying to defend, and where attackers might have come from, you might wander right to them.