The Tacoma Narrows Bridge was revolutionary - sleek and slender, with a roadway only 39 feet wide suspended 2,800 feet between towers. It was the third-longest suspension bridge in the world. It was also the most flexible.

Designer Leon Moisseiff had pioneered a new theory of suspension bridge design that emphasized elegance over stiffness. The Tacoma Narrows Bridge was his masterpiece - a ribbon of concrete and steel that seemed to float over the water. But Moisseiff's theory had a flaw. He had not adequately accounted for the effects of wind on a flexible structure.

From its opening day, the bridge moved. Even in moderate winds of 3-4 mph, the roadway would undulate in vertical waves, rising and falling as much as four feet. Drivers found it thrilling - the bridge became a tourist attraction. Workers bolted cables to the structure to dampen the motion. The motion continued.

Engineers were aware of the problem. The Washington Toll Bridge Authority hired experts to study the phenomenon. Solutions were proposed. Plans were drawn. But the bridge was new, and no one was in a hurry. The gentle waves seemed harmless. They were not.

On the morning of November 7, 1940, winds at the Tacoma Narrows reached 40 mph. The bridge began to move in ways it never had before. Instead of simple vertical waves, the roadway began to twist - one edge rising while the other fell, then reversing. The motion was violent, with the deck tilting as much as 35 degrees.

Authorities closed the bridge. The last car to cross belonged to Leonard Coatsworth, a newspaper editor. When the motion became too severe, Coatsworth abandoned his car and crawled on his hands and knees to safety. He left behind his cocker spaniel, Tubby, who was too frightened to follow.

For over an hour, the bridge twisted in the wind while spectators watched from shore. A local camera shop owner named Barney Elliott began filming. What he captured became the most famous engineering failure footage in history.

At approximately 11:00 AM, a section of the roadway buckled. Concrete cracked. Cables snapped. The main span - 2,800 feet of road and steel - tore itself apart and fell into Puget Sound. The side spans remained standing, their cables singing in the wind. Tubby the cocker spaniel went down with the bridge.

The Tacoma Narrows collapse transformed bridge engineering. Engineers learned that aerodynamic forces could cause 'aeroelastic flutter' - the twisting motion that destroyed the bridge. Future suspension bridges were designed with this phenomenon in mind, built wider and stiffer with features to reduce wind effects.

The bridge was rebuilt in 1950, wider and more rigid. A parallel span was added in 2007. The original towers still stand, supporting the new bridge. The wreckage of Galloping Gertie lies on the bottom of Puget Sound, now an artificial reef and a registered historic site. Tubby, the only casualty, was never recovered.