Thousands of passengers are stranded across Europe after a communications failure forced Greece to close its airspace, causing widespread cancellations and delays.
Officials are working to understand why radio communications were disrupted on Sunday morning, prompting the temporary suspension of arrivals and departures.
Some departures have since been allowed to resume - though inbound flights are still being told to divert or return to their point of origin. Athens' main airport is among the worst affected, while Thessaloniki airport has closed entirely.
The timing of the disruption has raised fears that travellers will not be able to get home before the start of the working week.
I was supposed to fly to London, via Stockholm, with SAS, one passenger at Athens International Airport told Greek broadcaster Mega TV. I'm working on Newport Hospital tomorrow morning. I'm not going to be there on time, I don't know what else to say.
Who is going to pay for the additional cost this travel will cost me? We still don't know if and when we're going to leave, no one is telling us anything.
More than 90 flights have been affected in and out of Athens airport alone - though some flights have since been able to resume.
Several flights from Dublin, Barcelona, and Paris were ordered to return to their airports of origin, while services from Copenhagen and Malta were canceled altogether.
The disruption comes at a busy time for air travel in and out of Athens, which typically sees over 600 scheduled flights per day, as people return from winter holidays.
The issue that caused the disruption is thought to concern the radio system used by air traffic controllers to communicate with planes in their airspace.
Italy, Turkey, and Cyprus are assisting Greece in managing the situation.
