Officials at the Chicago skyscraper insist no one was in danger.
The Willis Tower in Chicago has four transparent boxes that extend out from the building on the 103rd floor.
AP Photo/M. Spencer Green
"The Ledge" is a hugely popular tourist destination and since opening in 2009, some six million people have ventured into the boxes to admire the panoramic views.
The see-through bays are said to be able to withstand a weight of five tonnes.
AP Photo/Kiichiro Sato
On Wednesday evening, Alejandro Garibay, second from right, sat out in one of the boxes with his brother Ernesto, right, and cousins David Cazares, left, and Antonio Saldana.
The four men had headed up to the 103rd floor to watch a fireworks display.
However, while they were out in the box they heard an unnerving cracking sound.
AP Photo/Alejandro Galibay
Garibay told the Chicago Tribune: "I had my palms on the floor and I could feel it cracking. Honestly, I was in shock, in disbelief. I was scared."
Saldana told CBS Chicago: “All of us collectively start getting up, and it’s at that point that we hear and feel the glass start shattering. Mind you, this is happening in an instant. Incredibly fast.
"We see it completely just shatter all the way through, and at that point, all four of us just completely rushed right out of it.”
AP Photo/Alejandro Galibay
