Atlanta airport, the world's busiest, slowly returned to normal Monday after an 11-hour power outage caused by an underground fire stranded thousands of passengers in the southern US metropolis and unleashed a storm of criticism.

Electricity returned, but 400 flights were canceled and long lines choked airline counters at Hartsfield-Jacksonairport as carriers struggled to cope with a backlog of exhausted passengers.

Atlanta is the headquarters of Delta Air Lines, which said it had scrapped 300 flights, mostly morning arrivals, "to give the operation there an opportunity to more quickly return to normal."

Delta said it had to scrap about 900 flights on Sunday, but expected its Atlanta schedule would return to normal by Monday afternoon.

Power abruptly went out at the airport, which serves more than 100 million passengers a year, around 1:00 pm on Sunday and was restored around midnight.

Some passengers were unable to disembark from their planes for hours after landing, while others found themselves in darkened concourses, where even some water fountains stopped working.

Photos and videos from the airport showed people walking or sitting in crowded airport corridors, with windows only partially cutting through the gloom.

"There were a few emergency lights on, but it was really dark -- felt totally apocalyptic," said Heather Kerwin, who was in the terminal to take a flight for New York when the power went out.

"I decided to get the hell out of there," Kerwin told CNN.

A total of 1,180 flights to and from Atlanta were scrapped on Sunday, according to flight monitoring service FlightAware.

- 'Confusion and poor communication' -

Georgia Power CEO Paul Bowers said "switch gear" ignited in an underground facility, knocking out power to the airport and also a backup feed.

"We have our investigators in the tunnel," Bowers said, adding that foul play was not suspected.

"We are apologizing for this occurring."

Atlanta Mayor Kasim Reed also issued his "sincere apologies to the thousands of passengers whose day has been disrupted."

Anthony Foxx, a former transportation secretary under president Barack Obama, was among those who were stranded at the airport.

"We all understand that Snafus happen and most of the folks down here are doing their jobs to the best of their ability," Foxx tweeted.

"But, whatever the cause, it feels like this one was compounded by confusion and poor communication."

While stranded on the tarmac, he had written: "Total and abject failure here at ATL Airport today. I am stuck on flight, passengers and crew tolerating it. But there is no excuse for lack of workable redundant power source. NONE!"

Mike Vizdos told AFP that he and other passengers were trapped on their aircraft because the power went out just as it reached the terminal gate after arriving from Costa Rica.

"Sat on the plane for six hours and then cleared customs and immigration," said Vizdos, who was trying to get home to Richmond, Virginia.

"There are thousands of stories," he said.

Source: AFP