Earlier today, Delta Air Lines, Inc. grounded all flights nationwide–domestic and international–due to a power failure at its Atlanta data center. The outage lasted six hours ending around 8:40 a.m ET, although at that time Delta noted that only “limited” flights would be running and there would continue to be cancellations throughout the day.

.@Delta has cancelled approx. 300 flights due to power outage. Systems are coming back online, but delays and cancellations continue.

— Delta News Hub (@DeltaNewsHub) August 8, 2016

According to Ars Technica, which caught a post on the FlyerTalk forums, a forum for frequent-flyers, the outage appears to have been caused by a fire:

“According to the flight captain of JFK-SLC this morning, a routine scheduled switch to the backup generator this morning at 2:30am caused a fire that destroyed both the backup and the primary,” wrote walterD, a poster on the forums. “Firefighters took a while to extinguish the fire. Power is now back up and 400 out of the 500 servers rebooted, still waiting for the last 100 to have the whole system fully functional.”

Delta Airlines has posted an advisory about the flight outages on its website with information about rescheduling flights.

As of time of writing, Delta has cancelled over 650 flights and Delta CEO Ed Bastian appeared in a video to apologize for the outage, stating that “the Delta team is working very, very hard to restore and get these systems back as quickly as possible.”

This outage makes for the second massive-scale disruption to an airline company due to a data center issue. Last month, July 20, Southwest Airlines Co. suffered a router failure midday in the company’s Dallas data center. That failure led to the cancellation of 2,300 flights over four days as the company worked to recover from the outage.

Information about the Delta Airlines outage is still developing with no official statement from Delta as yet about the cause of the power failure or confirmation of the data center fire.

Data center fires are uncommon, but can be devastating to infrastructure. According to a 2015 report, 6 percent of all data center issues taht led to downtime were caused by fires. Large notable fires include a blaze in May 2015 that destroyed part of a roof and delayed a $2 billion dollar Apple data center in Mesa, AZ and one that affected thousands of British Telecom customers for over a day when a data center in Belfast, Ireland lit up. (More data center fires are listed in an article on the subject at IT WatchDogs from last year.)

Customers of Delta Airlines affected by the outage can call in to receive waivers or reschedule flights; but should expect long delays on the phone and long lines as the company works this out.

Featured image credit: Delta Connection Canadair CRJ 900; N554CA@LAX;04.02.2016 via photopin (license)