A California family was looking down at Chicago from 103 floors up when they heard the glass cracking.
"Crazy feeling and experience," Alejandro Garibay told NBC Chicago.
The family got off the structure and quickly notified building employees.
"I walked them over so they could see, and they were totally shocked,” Garibay began. “When we pulled our phones to start recording and take pictures, they asked us to leave right away."
A spokesperson for Willis Tower says there was never any danger because the glass didn’t crack, a protective coating did.
"This coating does not affect the structural integrity of The Ledge in any way,” said Brian Rehme.
“Occasionally, the coating will crack, as it is designed to in order to protect the surface of the glass," he concluded.
Opened in 2009, the four enclosures are called The Ledge, and those who dare can stand on them and look 1,353 feet straight down to the ground below.
Engineers say the ledges are made to hold up to five tons.
The other three observation points remain open while the fourth is repaired.