A sudden power failure at the world’s busiest airport that stranded thousands of passengers and snarled U.S. holiday air traffic underscores the vulnerability of the nation’s grid.

The 11-hour outage was caused by a fire in an underground electrical facility that also cut backup supplies to Hartsfield-Jackson Atlanta International Airport, said Southern Co.’s Georgia Power utility. Frustrated passengers were left scrambling in the dark while more than a thousand flights were canceled.

“This highlights two things,” said Paul Patterson, a utility analyst for Glenrock Associates LLC. “One, the grid is vulnerable. Secondly, it shows how dependent the modern economy is on reliable electric power.”

The airport blackout comes on the heels of hurricanes and wildfires that knocked out electricity service to millions of people from Florida to California. Earlier this year, power disruptions in New York City and San Francisco delayed commuters. Utilities say billions are needed to upgrade the nation’s aging infrastructure and make it more robust by investing in equipment sensors and other technologies that can be used to track and quickly resolve power failures.

The Atlanta incident “raises a question of whether a smart grid can help,” said Swami Venkataraman of Moody’s Investors Service. “Sensors can give you real-time information on important equipment across the grid that allows you to do preventative maintenance to avoid these type of issues.”

Fumes, Smoke

A piece of switchgear in an underground electrical facility failed and started the fire, which damaged cables serving the airport, Georgia Power said in a statement posted Monday. Firefighters had to clear dangerous fumes and smoke from the underground facilities before utility workers could gain access to carry out repairs.

Georgia Power Chief Executive Officer Paul Bowers apologized for the incident in a video posted on the utility’s website. “Our primary focus now is to investigate what happened and ensure that it never happens again,” Bowers said.

While most power at the Atlanta airport was restored around midnight, some operations had yet to return fully as of 9:30 a.m. The SkyTrain connecting the airport with an off-site rental car facility and the underground Plane Train linking concourses were both expected to begin running again around 10 a.m., airport spokesman Reese McCranie said.

Questions will be asked about why it took so long for Georgia Power to restore electricity, and state utility regulators say they will review the incident.

The blackout adds to a tumultuous year for Southern in which it wrote down billions after scrapping a first-of-its kind “clean coal” project in Mississippi and the fate of an over-budget and long-delayed nuclear project was thrown into disarray by the bankruptcy of a contractor. The utility will find out later this week whether it can complete the nuclear project in Georgia.

Southern fell 1.5 percent to $50.55 at the close in New York. The stock is up 2.8 percent this year.

“It’s not a good time for them to be high profile with a power failure,” said Kit Konolige, an analyst for Bloomberg Intelligence.