Buckingham Fountain was closed on Saturday after vandals dyed its pool water red, the Chicago Park District said.
The traditional gateway to "Summertime Chi" is now shut down until further notice.
The fountain's pool water was found tainted with red dye overnight. Pro-Palestinian graffiti was also discovered in and around fountain, located at 300 South Columbus Drive in Grant Park.
A crew member told ABC7 the fountain was more than halfway drained on Saturday afternoon. He was unsure when the fountain can be turned back on, and they were working on finding the source of the dye and making sure it doesn't stain the fountain's marble stone.
Buckingham Fountain vandalized, closed until further notice: officials
Until then, the fountain will stay off until further notice. The tourist destination will remain closed as crews continue to drain the fountain, which could take all day.
Drone pictures show what appears to be the coloring pumped into Lake Michigan along the lakefront.
"It's obviously an iconic spot, you know? It's fantastic! Look at this," tourist Tom Martin said.
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Martin, from Washington state, is visiting Chicago for the first time. The city landmark was considered a must see for him and his wife.
"Well, I was disappointed. We walked quite a ways over here to see it because my wife had told me about it and that it was beautiful," Martin said. "I'm somewhat sympathetic to what they're trying to accomplish, but this is not the way."
A city crew worker who did not want to appear on camera called the situation unfortunate, saying the city has never seen this level of concentrated dye poured into the fountain.
The crew member said it would take 16 hours to fully drain the million-and-a-half gallons of water. That process was more than halfway complete around 4:30 p.m. Then, the focus will turn to making sure that whatever the dye is, it does not absorb in the marble stone.
While efforts to find the source of the dye could take all day, police are working to find the culprit.
The crew member said luckily, the fountain was not on at the time, so the dye could not spread further and damage the water.
So far no one is in custody, police said.
"It's a beautiful city. It just is, and I hope the next time I come to Chicago, because I'll be back, I hope they got that fountain on," Martin said.