We're guessing the four-letter words were coming fast and furious from atop Willis Tower (formerly Sears Tower) on Wednesday afternoon.

A group of tourists checking out the Skydeck on the 103rd floor of the Chicago skyscraper received the shock of a lifetime when the glass pane they were standing on began to crack, NBC Chicago reports.

The photo of the cracked glass, taken by Alejandro Garibay, quickly went viral. Nobody was hurt.

Garibay told NBC Chicago that after the protective layer began to crack, he alerted staffers. "When we pulled our phones to start recording and take pictures, they asked us to leave right away," he said.

Tourists who go to the very top of the skyscraper are given the opportunity to step out onto "the Ledge," a series of glass boxes made of three layers of glass built to hold five tons, and then look straight down on the Windy City.

Willis Tower spokesperson Randy Stancik told DNAinfo Chicago that the tourists were never in any real danger.

Another spokesperson, Bill Utter, told the Chicago Sun-Times that this sort of thing has happened before.

Via the Chicago Sun-Times:

"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."

Stancik said that the piece of glass is cosmetic and is being replaced. Workers put a carpet over the cracked pane, "But nobody wants to go on it," he said.

No kidding?

Via DNAinfo Chicago:

"[The top layer of the Ledge] is designed to crack when somebody drops something on it," Stancik said. "It's not structural. The top piece is protective."

Willis Tower released a statement to Yahoo News:

"The Ledge was designed with a protective coating that completely covers all glass surfaces to protect against scratches. This coating does not affect the structural integrity of The Ledge in any way. Occasionally, the coating will crack, as it is designed to in order to protect the surface of the glass."

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"Skydeck Chicago is open today and operating as normal while the protective coating is replaced."

Regardless, the cracks do give a new meaning to the Skydeck's tagline: "Get out on the Ledge if you dare!"

Follow Mike Krumboltz on Twitter (@mikekrumboltz)