Have you ever been beneath the car deck of the Golden Gate Bridge? Beneath the girders? Do you even know what a girder is? The Obscura Society will show you, along with retired park ranger and historian John Martini, on a walking tour. The tour's highlight? An informational walk into a 120-year-old abandoned bunker that was constructed before the Spanish-American War. The bunker tour, combined with walks under the bridge structure and along the bluffs of Baker Beach, is a perfect Sunday outing for those interested in the obscure as well as the beautiful. The Obscura Society's mission is to bring modern explorers together on a continual journey to the world's odd and wondrous places, and this trip to an old Presidio bunker may be the perfect way to learn new things, take new Instagram photos, and meet like-minded bipedal individuals.