103-storey high glass floor cracks under visitors' feet

Visitors standing on a 103-storey lookout received a sky-high scare when the glass ledge appeared to crack below their feet.

While 412 metres above ground, those on the ledge of Chicago's Willis Tower SkyDeck watched as the glass below them shattered, fearing the cube would collapse with them on it.

Witness Jesus Pintado was there on Monday and captured the terrifying moment on video.

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

Thankfully it was just a protective layer covering the 1.3m-wide glass lookout that cracked, and nobody was in danger, Wills Tower spokesperson Terri Cornelius confirmed.

"It's like a screen protector," Ms Cornelius told the news outlet.

The glass coating was reportedly replaced late on Monday night, with the lookout operating as "business as usual" on Tuesday.

Despite the incident, a yoga session at the Skydeck is appearing to go ahead next week, according to a new Twitter post.

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