Employees are temporarily moving in to watch over the aquarium's 32,000 animals. They are keeping a close eye on a pregnant dolphin that's expected to give birth any day now.
Workers are bringing what they need to be there around the clock.
"Because of NATO there will be up to 60 of us staying through the weekend overnight," said Ken Ramirez, Shedd Aquarium. "That way, should there be an emergency, should there be a power outage, should we have a dolphin being born, should we have a medical issue. Any number of those things, we won't have to worry about, will we be able to get people here in time."
The Shedd will be closed to the public Saturday, Sunday and Monday.