The Golden Gate Bridge wasn't always destined to be the majestic, International Orange-colored span it is today. Back in the bridge's planning stage, the U.S. Navy was so concerned about its visibility that it lobbied hard to paint it black with yellow stripes. But that bold proposal looks absolutely mundane when you consider this one from 1932, which would've had the center of the structure submerged in the San Francisco Bay so ships could pass above.
The idea for a "boat tunnel" chopping right through the center of one of the world's now-most-recognizable bridges came from local inventor Cleve F. Shaffer, whose obituary claims he also "foresaw the tank, the bazooka, and the moving sidewalk, though he failed to win fame from any of them." The big attraction of Shaffer's plan was monetary: It was thought this bizarre structure would cost a mere third of the bridge's $35 million price tag. The authorities liked that perk enough that they were led to "consider seriously the erection of the boat tunnel bridge," reported the magazine Modern Mechanix, "which would be the only one of its kind in the world."
How would this Frankenbridge be constructed? The schematics call for regular truss spans at either end leading inward into sections floating on pontoons with interior ramps of the cork-screw-shaped variety you find at multilevel parking garages. Cars twisting down one ramp would wind up 45 feet below the waves in a 1,200-foot-long tube leading to the other ramp, where they would ascend into daylight and (presumably) make a fervent sign of the cross:
There are a couple things that might make a bridge builder blink twice at this thing. There's the sudden deceleration that would be needed to enter the ramp areas, for instance – a sure recipe for gridlock. Then there are the floating parts doing battle with the Bay's notoriously powerful currents. But old Cleve had thought up ways to fight the tides, according to Mechanix:
As a protection to the hulls and floating sections of the bridge, a resilient chain barrier would be hung from anchored buoys carrying lights and sound warnings. Because of strong tidal currents the floating units would have to be anchored in solid rock, and provision has been made for automatic motors to take up slack in the anchor lines. Underwater stresses are greatly reduced by streamline shape of the parts.
For whatever reason, the gutsy "boat tunnel" never made it out of the drawing room. Today, officials let shipping vessels pass through the Golden Gate the boring way: by steering them under it with the navigational help of booming fog horns.
H/t Modern Mechanix