CHICAGO, May 30 — A trip to the glass-bottomed observation deck at the Willis Tower yesterday left a group of tourists terrified.

The glass box observation decks on the 103rd floor of Chicago’s Willis Tower are a major tourist draw.

The glass cubes jut out of the building at a height of 1,353 feet and provide exceptional views of the city.

The glass balconies are designed to hold as much as five tons and the decks draw many thousands of visitors per year.

But yesterday a group of tourists from California got more excitement than they anticipated during a visit to the Willis Tower.

As they were photographing the night view, they suddenly heard the glass below them cracking.

Willis Tower officials said the glass cube skyboxes, opened to the public in 2009, are connected to the building by strong metal frames.

The glass bottom floor is made of three, one-and-a-half-inch thick glass panes which are coated on both sides with a special protective layer.

Building officials said it was the protective layer that shattered and the tourists were never in danger of falling.

Chicago City inspectors have closed the attraction until repairs are made, but said it does appear that only the protective layer was involved in the mishap. — Reuters