MIAMI — The world’s busiest airport suffered a major energy blackout that left thousands of passengers within its six concourses sitting in the dark, and thousands of others inside airplanes grounded on the tarmac.

A complete ground stop at Atlanta Hartsfield International Airport (ATL) was issued by the FAA, grounding all departing, and diverting all incoming flights—causing major chaos to Delta Air Lines’ biggest operation.

According to the airline, the power outage forced the cancellation of over 900 mainline and Delta Connection flights. The carrier said that more than 50 flights had been diverted to alternate nearby airports.

Southwest Airlines was forced to cancel about 70 departures out of 120, according to spokesman Brian Parrish. And other airlines with smaller operations, like American Airlines, canceled 24 flights into and out of the airport.

The power fault occurred past noontime on Sunday. Even though the cause is still unknown, a massive fire in the system’s underground tunnels affected the airport’s electric infrastructure, shutting down all its terminals for the rest of the day.

“The FAA tower can operate normally, however, departures are delayed because airport equipment in the terminals is not working,” the FAA said in a public statement.

As the concourses inside ATL blacked out, passengers had no option but to sit and wait for announcements made by the authorities. However, because of the lack of energy, this didn’t happen for several hours.

Atlanta Mayor Kasim Reed said that the fire broke out at a Georgia Power underground electrical facility, damaging two substations, as well as the backup system.

“I want to express my sincere apologies to the thousands of passengers whose day has been disrupted in this manner,” said Reed.

Atlanta’s Airport, in fact, recorded 104 million passengers last year, consolidating as the number-one in the globe. According to the airport’s website, over 275,000 passengers fly through its terminals every day with at least 2,500 aircraft movements.

“We certainly understand that the outage has caused frustration and anger, and we’re doing everything that we can to get folks back home right away,” he added.

The City of Atlanta deployed an emergency transportation squad from the airport to the Georgia Convention Center, helping stranded passengers leave the airport.

About seven hours after the blackout, power was restored at the airport’s newest Concourse F.

One hour before midnight, Georgia Power released a statement that it was still working closely with ATL to restore power.

Delta issued a statement in which it informed passengers that Monday’s schedule was expected to operate with normality, even though it advised them to check the airline’s website for further developments.

Moreover, the airline invited passengers to change their itineraries at no extra charge should flights go through the troubled airport.

This hasn’t been a good year for Delta in terms of operations. In January, the airline suffered a second IT meltdown, only six months after a more widespread, multi-day outage that, in August 2016, cost the company nearly $100 million. That situation alone forced the airline to cancel 2,300 flights over three days.

And only three months ago, in September, it was forced to cancel over 800 flights as Hurricane Irma flew by the metro area of 5.7 million people after wreaking havoc on the state of Florida.

Read More: Delta Cancels 800 Flights In Atlanta In Advance of Hurricane Irma

At 11:20PM, power had been restored to the airport’s main atrium, as well as in concourses T, A, B, and F.

— Developing story