Long closed to public access, Battery Townsley in the Marin Headlands has been refurbished and is now open to visitors the first Sunday of each month.
Come explore this labyrinthine fortification, and learn about San Francisco's most extensive -- and most secret -- World War II military fortification.
From 1940 to 1948, it mounted two massive battleship guns and housed more than 100 soldiers in an extensive network of underground tunnels; during the Cold War, it was used as an underground research facility.
Battery Townsley is reached via a moderately strenuous hike up the Coastal Trail, approximately one half-mile north of the Rodeo Beach parking lot.
Stay overnight at the historic Marin Headlands Hostel.