Visitors using Chicago’s Willis Tower Skydeck got more of a thrill than expected when the glass wall – which hangs hundreds of feet above the ground below – started to crack.

All four boxes situated on the building’s 103rd floor can hold five tonnes but were closed on Thursday (29 May) for the morning in what officials said were ‘routine inspections.’

Skydeck spokesman Brian Rehme said that while the damage may have looked worrying, the shattered glass was part of the coating designed to prevent scratches to the glass.

"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," said Rehme in a statement.

When the tourist experience opened in 2009, engineers said the boxes were composed of three layers of glass each about one-and-a-half-inches thick.