- The London Millennium Bridge is a pedestrian bridge that crosses the River Thames in London. It was completed in 2000 to commemorate the new millennium.

- Shortly after its opening, the bridge began to wobble when large numbers of people walked across it. This caused safety concerns and the bridge was closed for repairs.

- Investigations found that the bridge's swaying was caused by a phenomenon called resonance.

- Resonance occurs when an object's natural frequency is close to the frequency of an external force. In the case of the Millennium Bridge, the natural frequency of the bridge's deck was close to the frequency of the footsteps of the people walking across it.

- This caused the bridge to amplify the vibrations of the footsteps, resulting in the swaying motion.

- To fix the problem, tuned mass dampers were installed on the bridge. These devices vibrate in opposition to the bridge's motion, cancelling out the vibrations and reducing the swaying.