Atlanta’s Hartsfield-Jackson International Airport — the world’s busiest — is still recovering from an 11-hour power outage at the weekend that caused major flight disruptions, NBC News reports.

The blackout was caused by a fire that occurred about 1300 on Sunday, and power was restored around midnight. The toll on operations at the airport was extensive and compounded by foggy conditions on Monday morning.

On Sunday, 1,173 flights were cancelled and 207 were delayed due to the power outage. On Monday, the toll was 400 cancellations and 42 delays. The majority of the disruptions involved Delta Air Lines flights; Atlanta is a major Delta hub.

During the blackout, the US Federal Aviation Administration issued a complete ground stop at the Atlanta airport, meaning no flights could land or take off. Many passengers were stranded for hours inside planes on taxiways, until finally being allowed to deplane around 2230 on Sunday night.

The ground stop was lifted and the control tower at Atlanta was reopened after midnight, but ripple delays were expected throughout the system on Monday, as well as possible morning fog-related delays in Atlanta and a number of other US airports.