At first glance, the glass-observation boxes that jut out of the Willis Tower’s 103rd floor don’t look all that safe—and that is exactly the point. The SOM-designed attraction, known as the Ledge, opened in 2009 and offers “thrill seekers,” “death defiers,” and “people who can wait in  a really long line” the chance to step outside of the iconic skyscraper and look straight down at the streets of Chicago, 1,353-feet below. The floor of the suspended structure is comprised of 1.5-inch laminated glass panels, which can hold 10,000 pounds and withstand four tons of pressure. So, the danger is all imagined, right? Well, it certainly didn’t feel that way for a California family who visited last night.

When Alejandro Garibay and some of his family members stepped onto one of the boxes, they noticed cracks on the glass floor, which, remember, is  suspended 1,353 feet above ground. Garibay, told NBC 5 Chicago that it was a “crazy feeling and experience.” You don’t say!

According to Bill Utter, a spokesperson for the Willis Tower, there was nothing to fear. He told the Chicago Sun Times that it was only the coating that cracked and that the structural integrity of the Ledge was not impacted. “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.”

NBC 5 Chicago reports that the Ledge is closed today for what an official calls a “routine inspection.”