A glass ledge on the 103rd floor of the tallest skyscraper in Chicago cracked under visitors’ feet on Monday night.

The all-glass area of the Willis Tower, in which sightseers can view the city of Chicago from a high point, is called the Skydeck. Its protective top layer is made of laminated glass, which is built to maintain its integrity even while it cracks.

On Monday, it cracked into “thousands of pieces,” according to WBBM.

In a video shared on Facebook, visitors appear nervous and scared, and immediately huddle away from the glass area.

The Willis Tower, which was called the Sears Tower until 2009, said no one was ever in danger.

The layer is known as the “Ledge,” and was constructed in 2009, after Tower officials noticed “the hundreds of forehead prints visitors left behind on Skydeck windows every week.” Build to hold five tons, the attraction give tourists and residents an immersive view of the midwestern city.

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An incident in 2014 on the same area caused a closure of the entire Skydeck. The Willis Tower said no one was hurt in that incident either.

Still, tourists appeared frightened, as the incident was, of course, not supposed to happen.

“There was a woman with two kids and they looked really pale and scared because the floor just cracked,” witness Jesus Pintado told WBBM.

“I’m scared of heights in general so when I saw that happen, I was like nope, not going on,” Karly Pintado, who was also on the 103rd floor that day, said.