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A California family visiting the Willis Tower in Chicago said they heard the glass flooring on its 103rd floor Skydeck begin to crack Wednesday night while they were taking in views of the city.
A spokesman from the building said the sound came from the coating on the glass and the family was never in danger, The Chicago Sun-Times reported.
"Occasionally this happens, but that's because we designed it this way," he told the paper. "Whatever happened last night is a result of the protective coating doing what it’s designed to."
Moments before 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."
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.
There are four glass enclosures called The Ledge, which opened in 2009 and give tourists the feeling of standing suspended in air.
The balconies are suspended 1,353 feet above the ground and jut out 4 feet from the building. They're actually more like boxes than balconies, with transparent walls, floor and ceiling. Visitors are treated to unobstructed views of Chicago from the building's west side and a heart-stopping vista of the street and Chicago River below -- for those brave enough to look straight down.
All four boxes were closed Thursday morning for what a spokesman called a "routine inspection" and said the tower hoped to reopen the enclosures shortly, NBCChicago.com reported.