On June 10, 2000, tens of thousands of Londoners crossed the Millennium Bridge for the first time, a new structure connecting St. Paul's Cathedral to the Tate Modern. Designed by Foster + Partners, with engineering from Arup and contributions from sculptor Anthony Caro, the bridge cost 18.2 million pounds. It was nicknamed the "blade of light" for its slender design. On the opening day, it is estimated that up to 100,000 people gathered to cross the bridge, with a maximum of 2,000 people present on the bridge at any given time, corresponding to a density of 1.3 to 1.5 people per square meter according to Arup's measurements. The bridge began to sway on its opening day due to the synchronized walking of pedestrians. Each step exerts a lateral force of about 25 newtons, and when multiplied by hundreds of feet, this created a movement of 70 millimeters in amplitude. The strongest oscillations affected the southern span, around 0.8 Hz, and the central span, between 0.5 and 1.0 Hz. The lateral acceleration reached up to 250 milli-g, a threshold sufficient for many passersby to hold on to the railings rather than continue walking normally. On Sunday, June 11, the managers reduced the number of people allowed on the bridge at once, but the movement continued. On June 12, 2000, the bridge was closed for an in-depth analysis of the phenomenon. The nickname "wobbly bridge" quickly became associated with the structure. Although no one had instructed pedestrians to march in step, each person unconsciously adjusted their stride to the slight swaying of the ground, an almost instinctive reaction. This gesture, repeated by hundreds of people at the same time, transformed a minimal oscillation into a noticeable movement. According to the Japanese Failure Knowledge Database of the University of Tokyo, each step exerts a lateral force of about 25 newtons, a negligible push when considered individually, but formidable when multiplied by hundreds of synchronized feet. Arup demonstrated that this phenomenon was not unique to the Millennium Bridge, and the key term used by the engineers was "synchronous excitation," a mechanism that can affect any pedestrian bridge whose lateral frequency drops below approximately 1.3 Hz, provided that a sufficient number of pedestrians are present at the same time. A precedent existed: in 1975, a road bridge in Auckland, New Zealand, experienced comparable lateral movements during a demonstration with between 2,000 and 4,000 people. Closing the bridge was only the first step. Engineers conducted crowd tests directly on the structure, with a first series of 100 people in July 2000, followed by a second in December. A detailed report came out in September 2000, based on three distinct university studies. The solution involved combining 37 viscous dampers, similar to those in a car's suspension, and 54 tuned mass dampers, designed in Germany to absorb the energy of the swaying by oscillating in the opposite direction of the deck. The renovation project, from May 2001 to January 2002, cost 5 million pounds, funded by the Millennium Bridge Trust. More than 2,000 people participated in the final tests before the reopening. On February 22, 2002, the Millennium Bridge reopened to the public, with no swaying reported. British press noted the complete absence of perceptible movement during the final tests, a striking contrast with the chaos of June 2000. The episode has permanently marked the engineering of pedestrian bridges, influencing construction standards worldwide and transforming a high-profile failure into a technical reference. The bridge now welcomes several million crossings each year, without the 2000 phenomenon having repeated itself.