Thousands of passengers are stuck across Europe after Greece closed its airspace on Sunday morning. The closure came after a radio communication system used by air traffic controllers suddenly stopped working. The failure forced officials to halt all arrivals and most departures. Athens International Airport and Thessaloniki Airport are hardest hit, with Thessaloniki closed completely. More than 90 flights in and out of Athens alone were affected. Some outbound flights have resumed, but inbound flights are still being diverted or returned to their origin. Flights from Dublin, Barcelona, and Paris were turned back, while cancellations hit routes from Copenhagen and Malta. Some arrivals are being diverted to Turkey. Passengers face uncertainty and delays. One traveler at Athens airport said, "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." The disruption comes as Greece sees over 600 flights daily during the busy winter holiday return season. A staff member at Athens airport told the BBC that 35 planes per hour are now allowed to take off, but inbound flights remain blocked. The issue seems linked to the failure of an antenna in the Gerania Mountains near Athens. Panagiotis Psarros, chair of the Association of Greek Air Traffic Controllers, said, "We could not communicate with aircraft in the sky." Italy, Turkey, and Cyprus are helping Greece manage the crisis. Domestic travellers may try rail transport, but it will take longer. Officials are investigating to restore normal flight operations soon.