The Shedd Aquarium was closed this afternoon after power went out in the building along the lake, officials said.
The electricity failed around 1 p.m., and patrons and staff were evacuated as a precaution said Shedd spokeswoman Andrea Smalec.
By about 1:50 p.m., officials said some of the power to the building had been restored. Officials are investigating the cause.
Patrons are being offered vouchers to return, said Smalec.
The aquarium is expected to reopen during its normal hours Friday, said spokesman Roger Germann.
Germann said about 2,000 people were evacuated as a safety precaution after the power went out to a portion of the building around 12:30 p.m.
Germann said none of the aquarium's 32,500 animals were affected, as back-up generators kicked in right away.
"We have a lot of redundant systems so those kicked in right away," Germann said.
Germann said the outage is attributed to an internal power issue and that aquarium officials are working with ComEd to find out more about what caused it.
Aquarium staff turned away approaching visitors and handed bottles of water and free passes to guests who were asked to leave the building.
"There was no alarm or anything," said Valerie Gordon, who was in the food court with her daughter when staff asked her to exit the building. "It was real organized. ...Everybody was pretty calm."