O'Hare has seven runways. On Thursday, starting just before 9:30 p.m., the power went out on four of them. The Department of Aviation says the problem was fixed about 40 minutes later. But what the city doesn't know is what caused the lights to go out.
Aviation officials say that there are fewer flights that time of night. But the outage did affect some flights that had to delay their landings.
The past couple days at O'Hare have not been easy for people flying because of the rainy, windy weather. There have been hundreds of cancellations and delays.
This isn't the first time a Chicago airport has had trouble with its runway lights. Last June, Midway's runways lost power for about four hours.
The Department of Aviation has not said exactly how many flights were impacted by the outage Thursday night.