America's highest glass observation deck cracks under foot, shocking holidaymakers.

The Willis Tower's glass Skydeck is a favourite destination for thrill-seekers in Chicago, but tourists standing on the glass-bottomed observation deck got an extra shock when large cracks began to appear beneath them.

The clear protective layer that allows visitors to look down on the city below shattered into tiny fragments.

High anxiety: The observation deck of the Willis Towers is not for the feint of heart. Photo / Getty Images
High anxiety: The observation deck of the Willis Towers is not for the feint of heart. Photo / Getty Images

Suddenly the distance between observers and the 412m drop seemed too close for comfort.

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Talking to CBS Chicago, Jesus Pintado said the cracking floor was more than the visitors were bargaining for.

"There was a woman with two kids and they looked really pale and scared because the floor just cracked," he said.

The Willis tower said no tourist were put in danger, as the protective layer had performed its task in stopping any material (or tourists) from dropping the 103 storeys to the streets below.

SkyDeck Crack in the USA

NO THANKS >> Cracks have appeared in the SkyDeck ledge of Chicago’s Willis Tower -- 103 floors up -- for the second time since 2014!

Posted by FIVEaa on Wednesday, 12 June 2019

However the damage to the attraction has certainly spoilt its appeal, in which tourists appear to walk out over thin air on a thin windowed ledge.

A spokesperson for the Willis Tower spoke to Indiana broadcaster WTHR saying:
"The protective coating, which acts like a screen protector for The Ledge, experienced some minor cracking.

Perfectly safe: Children peer through the glass floor of the Skydeck of the Willis Tower, Chicago. Photo / Getty Images
Perfectly safe: Children peer through the glass floor of the Skydeck of the Willis Tower, Chicago. Photo / Getty Images

"We replaced the coating last night, and The Ledge is open for business as usual."
About one and a half million tourists are attracted to the ledge each year.

Each pane in the shelves are comprised of three layers of half-inch thick glass.

And though it is designed to hold five tons, the protective layer is designed to stop fractures appearing in the structural panes.

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It's not the first time visitors have had such a shock. The layer cracked once before in May 2014.

Originally called the Sears tower, the Willis building was the tallest in the world from 1973 to 1998. It was subsequently overtaken by the Petronas Twin Towers in Kuala Lumpur and the Burj Khalifa in Dubai. However it is unrivalled for it's views out over midtown Chicago and Lake Michigan

The four shelves were opened to the public ten years ago, in 2009.