It is what everyone who stands out on a glass floor at the top of a skyscraper fears the most.
And this week it almost came to pass when a family visiting the Skydesk Ledge at the Willis Tower in Chicago suffered the scare of their lives when the glass floor of the tourist attraction 412m up cracked as they sat down for a picture.
The astonishing incident caused staff to hurriedly evacuate them as the distinct sound of shattering glass could be heard creeping across the glass enclosure that juts out from the 103rd floor of the building.
"I had my palms on the floor and I could feel it cracking," said Alejandro Garibay, who was posing for pictures at the time with his brother and two cousins. "Honestly, I was in shock, in disbelief. I was scared."
"One of the craziest feelings I had in my life," added his cousin, Antonio Saldana. "I thought I was going to fall through."