When London’s Millennium Bridge opened in 2000, it began to sway unexpectedly under the footsteps of the inaugural crowds. The danger of vibration in bridges was well known — the Albert Bridge (below) still bears a sign dating from 1873 that warns soldiers to break step while crossing. But the Millennium Bridge revealed a new phenomenon: As the pedestrians felt the bridge swing beneath them, they altered their movements to maintain their balance and began to sway together in step. This increased the amplitude of the bridge’s oscillations, a factor called synchronous lateral excitation.

The bridge was closed on opening day and underwent $9 million in repairs to increase its damping. It reopened in February 2002 and is now free of vibration, but it’s still known affectionately as the “wobbly bridge.”