The outage, which lasted several hours and affected most of Greece's airports, stranded thousands of travelers. Authorities have ruled out a cyberattack, but the cause remains unknown - and officials admit the systems didn't get fixed, they simply came back on their own.

"Suddenly communications went down. You could only hear a high-pitched whistle," said one controller, speaking on condition of anonymity. "The thing is, we don't know what caused it and how it ended. We want to find out the exact cause to ensure once and for all that this will not happen again."