Family members inside a glass viewing enclosure at Chicago's tallest building, 412 metres above the city, had a moment of terror as the see-through floor appeared to crack beneath them.
Alejandro Garibay, his brother and two cousins heard cracking when they were on the tourist attraction known as The Ledge, which juts out 1.2m on the west side of the 103rd floor of Chicago's Willis Tower.
They stepped off and alerted staff members.
"I walked them over so they could see and they were totally shocked and asked us to step away and then proceeded to start calling staff and techs," Garibay told NBC Chicago.
"When we pulled our phones to start recording and take pictures they asked us to leave right away."
It was a "crazy feeling and experience," he said in an email.
A spokesman for Willis Tower said the only thing that cracked in the Wednesday night (local time) incident was a protective coating on the top of three layers of glass, each 1.3 centimetres thick. The coating had done exactly what it was supposed to.
The coating had cracked before, generally when a visitor had something sharp in his pocket or hand that shattered the coating.
Buildings Commissioner Felicia Davis said the public was never at risk. "The integrity of the structure was never compromised."
All four boxes of The Ledge were closed for a routine inspection.
The Willis Tower, formerly known as the Sears Tower, is the eighth-tallest building in the world and the second-tallest in the United States.