In what could have been a nightmare come true, part of the glass floor in a viewing box high above Chicago cracked right below a group of California tourists. But it was apparently much less of a problem than it might have seemed.

Alejandro Garibay and his family were taking in the sights at Chicago's Willis Tower on Wednesday night. The tower's Skydeck, which opened in 2009, is on the 103rd floor, about 1,353 feet above Chicago's downtown.

Right before stepping on the glass, Garibay and his family were joking with Willis Tower staff about the safety and durability of the glass.

"They jokingly and confidently responded, 'It's unbreakable,' so we just went on," Garibay told the NBC affiliate station in Chicago.

While on the glass, Garibay heard cracking. He and his family told the staff, who directed them away from the glass box.

A spokesman says that's just a protective coating on the Willis Tower ledge that cracked. http://t.co/7Z7uiK7wRR pic.twitter.com/Yj7NYiBEFy — Chicago Sun-Times (@Suntimes) May 29, 2014

"When we pulled our phones to start recording and take pictures they asked us to leave right away," Garibay said.

A building spokesperson told the Chicago Sun-Times that it was the protective coating that cracked.

“Occasionally this happens, but that’s because we designed it this way,” he said. “Whatever happened last night is a result of the protective coating doing what it’s designed to."

The Skydeck, which provides panoramic views of the city, was open on Thursday.