The ball, which bore an image of the Chicago skyline and the signature of actress Jenny McCarthy, was stolen during the weekend from the spot where it was placed alongside two other balls, bearing images representing the Chicago Bears and the Chicago Blackhawks, the Chicago Tribune reported Tuesday.
"We are upset about it, of course,'' exhibit spokesman Eric Schmidt said. "It's an empty spot, so we're certainly disappointed. We did everything to try and secure them as best we could.
"They weigh about 30 pounds but it takes a little working to get them out of the base," he said. "The most disappointed person is the artist, Jeff Budzban."
The two remaining balls were initially removed from the downtown site just north of the Michigan Avenue bridge north of the river, but the Blackhawks ball was later returned with chains securing it in place.