Officials at Chicago's Willis Tower say the popular tourist attraction is safe, even though a glass ledge jutting from the building's 103rd floor appeared to crack beneath the feet of a visiting family.
The see-through glass bays are known as The Ledge and extend about 1.2 metres from the building, which was once called the Sears Tower.
Officials say the family wasn't in danger when the cracks appeared on Wednesday on a glass enclosure.
In a statement, Willis Tower says a protective coating covering the glass surface cracked, not the glass itself.
They say the cracks don't affect the “structural integrity” of the ledge, which is designed to occasionally crack.
The ledges have proven to be a popular - if a frightening - experience since opening in 2009.
Fox News reported that as the family walked out onto the Skydeck, they say they were reassured by staffers that the window flooring was unbreakable.
"I had my palms on the floor and I could feel it cracking," Alejandro Garibay, 23, told The Chicago Tribune. "Honestly, I was in shock, in disbelief. I was scared."
Mr Garibay was watching fireworks at the Navy Pier.
"It was an awesome view. We were getting up and walking away and for some reason I thought it would be cool to get a picture of my cousins and brother ... Then we started getting off and, as we push ourselves off, I could feel it cracking," he said.