Atlanta's airport, the busiest in the world, was hit by a power outage on Sunday that caused hundreds of flights to be delayed or canceled.
"Hartsfield-Jackson Atlanta International Airport sustained a power outage shortly after 1:00 pm today," the airport said in a statement.
The Federal Aviation Administration announced that it had put a "ground stop" on flights inbound for Atlanta, meaning they are held at the airports from which they were to depart.
"The FAA Tower can operate normally, however, departures are delayed because airport equipment in the terminals is not working," it said on Twitter.
Theories for what might have caused the outage circulating online included a power line cut by a construction crew and a fire near the intra-terminal train.
But the airport said that "the cause of the incident remains under investigation."
Flightview.com, which tracks air travel data, listed hundreds of flights as being delayed or canceled, and a bar graph on the site showed more than 80 percent of departing flights at the airport and 80 percent of arriving flights as being late.
Another aircraft tracking site, flightstats.com, also reported widespread Atlanta delays.
Photos and video posted on social media showed people walking through darkened corridors at the airport, with windows only partially cutting through the gloom.
Those stuck at the airport were predictably not amused.
"I haven't eaten since 8AM central. I feel great. #AtlantaAirport," one user tweeted.
Others said they were waiting on the tarmac: "Atlanta airport doesn't have power. I'm trapped on the runway, I'm hungry, and slightly buzzed..... Send help!!!"
And some said people were not being permitted to leave the airport, including a mother who tweeted: "My kid is trapped with the others at the Atlanta airport. No one is allowed to leave. Isn't this kidnapping?"