Like something straight from every acrophobic's nightmare, one of the glass enclosures off the 103rd floor of the Willis (nee Sears) Tower in Chicago shattered yesterday while a family was standing on it.

"Crazy feeling and experience," Alejandro Garibay wrote in an email to NBC Chicago. Garibay said that, before he stepped on the enclosure, some of the staffers were joking about how the glass box—called The Ledge—was unbreakable

"They jokingly and confidently responded, 'It's unbreakable,' so we just went on," he said.

Not long after, Garibay said he, his brother, and two cousins heard the glass breaking. They quickly stepped off and and notified a Willis Tower employee

"I walked them over so they could see and they were totally shocked and asked us to step away and then proceeded to start calling staff and techs and I don't know who else. When we pulled our phones to start recording and take pictures they asked us to leave right away," he said.

The glass enclosure, one of four attached to the Willis Tower, is composed of three layers of glass, each about one half-inch thick, and was designed to hold up to five tons.

Officials from the Willis Tower said they'll release a statement about the incident later on Thursday.

UPDATE: From NBC Chicago:

A spokesman for Willis Tower said that what cracked was a protective coating on the glass structure and not the glass itself.

"This coating does not affect the structural integrity of The Ledge in any way. Occasionally, the coating will crack, as it is designed to in order to protect the surface of the glass," said Brian Rehme.