The soldiers are gone, but certainly not their history.
"The space that we're looking at right now is the interior line of defenses of the system," says Matthew Kent, author of a field guide highlighting the harbor defenses of San Francisco -- the series of bunker-like buildings stretched from Point Reyes in the north to Half Moon Bay in the south.
Kent wants people to remember these old batteries.
"I think what really sparks people's interest is that, hey, these were built prior to 1900," he says. "You're actually looking at structures that are still within the city, you know, pre-1906 that are still standing."
Most of the harbor defense sites were used through World War II and then turned over to the National Park Service in the early seventies.
"Going back as far as when the Spanish first got here, there was one series of fortifications after another -- all built not because a war was going on -- and we had to fight back, but built in anticipation that someday war might come," says retired park ranger John Martini who now leads tours through one of the largest batteries on the first Sunday of each month. "Battery Townsley was a coast artillery battery contructed on the eve of World War II and it mounted two battleship-sized guns that could fire shells weighing over a ton, 25 miles out to sea."
Many of the batteries are accessible to hikers on the outside, but sealed off from visitors inside. Martini gave ABC7 a tour of Battery Townsley in the Marin Headlands.
"The thickest area we are just about under is probably about 25 to 30 feet of earth above us and the concrete averages about 4 to 6 feet thick all over," he says. "It was really built literally to be bomb-proof."
Despite all the effort to build these massive batteries, the guns were only ever fired in practice maneuvers.
Up to 100 soldiers would call this place home.
"It was crowded, it was stuffy and it was cold," says Martini.
When it closed, Battery Townsley was used for explosives testing and as a training ground. Since 2005, Golden Gate National Recreation Area, the Golden Gate National Parks Conservancy and many volunteer groups have been working hard to preserve it.
Despite all the efforts to keep people out of the batteries -- drug dealers, gang members, and curious teens used many of them as hangouts.
"Inside of this place was party central for years," says Martini.
It was in bad shape when volunteers first started working on its restoration.
"The floor was covered with garbage and debris," he says. "There was graffiti on every vertical and a lot of the overhead surfaces."
After two and a half years and about $150,000, Battery Townsley opened to the public in 2007. The park service is now working on restoring other battery sites.
"It's important to save these because they are part of our national heritage -- our national story," says Martini.
A local urban legend is that the old batteries are linked by a series of secret tunnels. Martini says that's not true.
- About Battery Townsley (National Park Service)
- "Harbor Defense of San Francisco – A Field Guide" by Matthew Kent
Written and produced by Ken Miguel