Perhaps every driver has a greatest fear crossing a bridge.

Imagine yourself on your daily commute to work, dropping the kids off at school or driving to work. You’ve always thought that your route consisted of crossing a long bridge over a river and never stopped to think about it. We’re sure there is some subconscious fear in your head that something might go wrong while you’re crossing the bridge. It’s rush hour and there’s nowhere to go in case something happens. A mad dash to get to the other side of the bridge would be disastrous for everyone involved. That fear quickly became a reality in China’s Guangdong province, when the bridge they were on began to move up and down.

The suspension bridge spans the Pearl River, and is approximately 2.25 miles long. It was constructed in 1997 and connects the Nansha District, Guangzhou, to Humen Town, Dongguan. Safety officials who monitor traffic cameras saw the incident on May 5, and were able to spot it. The bridge was then cleared and closed indefinitely to prevent further damage. Popular Mechanics have now taken up the story and offer a more technical explanation of why this happens. Engineers allow for slight flexing in bridges, much like a skyscraper. Most structures allow for some swaying but this tolerance is kept to a minimum, so it’s not easily measured.

Because they are less streamlined and offer little wind resistance, suspension bridges are especially wind-tolerant. The bridge deck was covered with a 1.2-meter high retaining wall. This created a large sail. The wall is blown by the wind and creates an air vacuum. This pulls the bridge’s surface up and down. The stronger the wind, however, the greater the impact. The retaining wall was not damaged and no injuries were reported. It is unknown if people will use the bridge again.