Authorities in Cologne closed off a large section of the city center Wednesday, evacuating homes and businesses and shuttering tourist spots, as experts worked to defuse three unexploded mega-bombs from World War II.

Tens of thousands evacuated from German city on Wednesday after wartime bombs were discovered, in largest such operation since the end of WWII.

Authorities in Cologne said this was the largest evacuation of the city since World War II. Unexploded ordnance has commonly been found across Europe.