Atlanta's Hartsfield-Jackson International Airport was been hit by a power outage that disrupted flights Sunday at the world's busiest airport.

The issue first hit the airport around 1 p.m. ET on Sunday. At least one concourse had power restored by 7 p.m., though the airport said it could take until midnight before power was restored airport-wide. The airport added that "Georgia Power believes the issue may have involved a fire which caused extensive damage in an underground electrical facility."

Whatever the cause of the outage, it was snarled flights. Around 1,150 flights had been canceled there as of 7:45 p.m., according to flight-tracking service FlightAware. That accounted for close to half of the entire day's flight schedule. And of the flights that had not been canceled, hundreds more were running late.

Atlanta TV station WXIA Channel 11 reported that as of 8:30 p.m. ET, Delta alone had canceled 900 flights and was still deplaning passengers. Atlanta is the busiest hub for Delta, which runs a huge operation there.

For much of the afternoon and evening, the Federal Aviation Administration had issued a ground stop for flights scheduled to take off for Atlanta.

U.S. Customs and Border Protection announced that international flights were being diverted to other airports.

Major airlines were canceling flights. Southwest Airlines canceled "all but a handful of international flights," according to an airline spokesperson. The FAA told WXIA that as of 3:22 p.m. ET, between 80 to 100 jets are parked on the taxiways and waiting for gates.

As frequently happens in such cases, passengers took to social media with details of how the issue was playing out.

"Just landed in ATL and apparently the power is out in the airport so we have to wait on the plane until the airport regains power," one said in a tweet from 1:21 p.m. ET.

"Power is out at Atlanta airport and no info being shared with people in the terminals... we're literally in the dark," another added at 1:18 p.m. ET.

"Entire @ATLairport out of power and it is a total madhouse but thankfully @Delta skyclub still pours a stiff drink and continues to serve. Cheers!," another added at 1:19 p.m. ET.

It was not immediately clear how many flights were affected, but a prolonged problem could cause major problems at the airport. Atlanta handles more than 270,000 passengers per day, on average.

The airport is a major hub for Delta Air Lines. Southwest, the second-busiest carrier there, also has a significant base there. Sunday's problems could ripple out and affect flights at other airports, depending on how long it takes to resolve the issue.