Galloping Gertie

On November 7, 1940, the Tacoma Narrows Bridge tore itself apart in a 64 km/h wind — not a hurricane, just a moderate gale. The bridge did not simply sway.

The culprit was aeroelastic flutter - wind hit the flat deck at just the right angle to create a self-reinforcing twist cycle. Each twist changed the wind's angle, amplifying the next. The flutter fed on itself until the deck tore apart.