During WWII, German forces occupied Greece for years. Cut to 2017, and a gas station in Thessaloniki (a city in Northern Greece) was doing some work this week to expand fuel storage tanks, and found one big, huge, explosive surprise. An undetonated, 500-pound bomb.

It is presumed to be one of the biggest wartime bombs found in urban Greece, and has caused one of the country’s largest mass evacuations in recent history. Tens of thousands of people in Thessaloniki have been evacuated, while government officials deal with the 500-pound war bomb.

The evacuation is disrupting not only the day to day in the area, like bus and train services, but a refugee camp with nearly 500 individuals as well. They’re among those being relocated to schools, sports halls, and cultural centres.

The bomb is in too bad a condition to determine if it’s German or Allied, but some people have speculated to the Associated Press that  British and US planes dropped the bomb on Thessaloniki  in 1944, in an attempt to disrupt German rail facilities, and it just didn’t go off.

Military officials are saying this is a very complex operation, and the first of its kind in a densely populated area of Greece. Hundreds of people are involved in the operation. They plan to defuse its detonator, bring it back to a military base, and figure out how to neutralize it there.

Thessaloniki’s Deputy Governor Voula Patoulidou has said, “It is the first time something like this is happening in Greece.”