Of the four tunnels that penetrate San Francisco's hilly geography, the Broadway is the newest and the ugliest. Completed in 1952 at a cost of $5 million, it has none of the Edwardian charm of the Stockton, the Duboce, or the Twin Peaks -- all engineered by M.M. O'Shaughnessy during the city's great post-quake rebuilding-and-expansion era. The Broadway's two enormous façades are bleak Cold War concrete; its interior resembles a decrepit Formica restroom; and its overall ambience is starkly utilitarian. So why is it the great San Francisco tunnel? Well, it makes possible the unique experience of departing the burbs west of Larkin Street and emerging a few moments later in the teeming excitement of honest-to-God San Francisco -- North Beach on the left, Chinatown on the right, and the Embarcadero straight ahead. The Broadway's open to pedestrians, offering a handy shortcut for those of us who don't relish the Alpine topography between Leavenworth and Taylor. A distinctly San Francisco touch is the welcoming presence of the Hyde Street cable car chugging along at one end of the tunnel and the Mason Street cable car at the other. And there's something undeniably, well, Kubrickian about staggering through this curving, snakelike monolith after a night of North Beach revelry, the harsh neon splashing off the stark yellow tiles, the whoosh of the occasional automobile echoing up and down the narrow passage, the weight and immensity of Russian Hill looming above.