Delta Air Lines suffered a massive systems outage this morning that left all its flights worldwide grounded. After several hours, flights were slowly and in a limited way able to start taking off again, but passengers all over the map are still facing massive delays.

At 1:30 p.m. Eastern time, Delta said it had cancelled 451 flights outright, and that 1,679 of its 6,000 flights for the day had been able to operate. Looking at FlightAware’s live map for the airline does indeed show dozens of planes in the air, on their way to where they need to be.

But the backlog for remains hours long and thousands of customers deep, and customers will continue to experience delays while the airline tries to get everything up and running once more.

In a video posted to Delta’s website and social media accounts, company CEO Ed Bastian apologized to customers for the continuing chaos.

Much like Southwest’s executive leadership did a few weeks ago, Bastian recorded the video in the airline’s Operations and Customer Center, where Delta employees were hard at work in the background trying to solve a thousand problems at once.

“I apologize for the challenges this has created for you, for your travel experience,” Bastian said. “The Delta team is working very, very hard to restore and get these systems back as quickly as possible.”

“I appreciate the hard work the Delta team and the Delta people are doing to bring our system back up as quickly and safely as possible, and once again apologize for any inconvenience this has caused to you, our customers.”

So what actually happened, that caused such a catastrophic failure?

Delta said a power outage in Atlanta, the airline’s home city, caused the systems to go offline. However, the Washington Post asked the local power utility, and they say it wasn’t them.

“We believe Delta Air Lines experienced an equipment outage; other Georgia Power customers were not affected,” a spokesman for the utility told the WaPo, adding that Georgia Power had sent over staff to assist Delta in getting back online.

An aviation expert told CNNMoney that delays were likely to continue at least until Wednesday, due to the man-hours problem. Crews have to get a certain amount of rest in a 24-hour period, and that means that when the planes are finally ready, the staff to get them airborne might not be.

“You have airplanes and crews out of place. You’ve got passengers all over,” an expert told CNN. “This is something you can not completely prepare for.”

As the AP points out, though, it might be exactly the sort of mess airlines should start preparing for… because basically everyone has had one. Southwest went out just a few weeks ago. American had a big one in 2015, as did United, which also went down for a morning in June. And that’s not even counting the dozens of incidents large and small that happened in 2014 and earlier, or the fact that air traffic control itself is just as subject to tech outages.

The computer systems that make modern air travel work are huge, efficient, helpful, and largely automated — but their sprawling, hands-off nature can compound misery when something goes wrong, as it did today. And, as another CNNMoney story points out, those systems are always going to be subject to some kind of error. Power will always fail. Some human somewhere will always screw up. Some squirrel will chew through some wire.

That’s what backup systems, disaster recovery plans, and off-site redundancies are for, an expert told CNNMoney. As to why they didn’t work this time, well, someone at Delta probably wants to know as badly as all the stranded passengers do.

“You’re basically saying, ‘We had power failure in a location but unfortunately we were unable to continue operations from a secondary data center despite the fact that we spent hundreds of millions of dollars on it,'” a disaster recovery expert told CNN. And someone at Delta is going to want to know where that money went.

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