The Ledge closed all of its four glass boxes at the top of Chicago’s Willis Tower, after the glass floor of one of the boxes appeared to crack beneath the feet of a group of tourists.

Alejandro Garibay was in one of the glass enclosures along with his brother and two cousins when they saw the glass crack beneath them.

The group told Chicago Tribune that they jumped off the platform as soon as they spotted the cracks appearing, fearing that they may fall 103 storeys to the ground below.

Ledge at Willis Tower Skydeck shows cracks. Our post updated w/ statement from tower official http://t.co/vEoG1p8QFI pic.twitter.com/1FZwV0rh2q

— NBC Chicago (@nbcchicago) May 29, 2014

Skydeck Chicago have said that the closure is for a routine inspection, but were keen to point out that it was the “scratch-proof” coating that cracked, and that this had no impact on the structural integrity of the ledge box, which is comprised of three layers of glass and can hold five tonnes.

The cracked coating was replaced on Thursday afternoon.