Experts from the Kampfmittleräumdienst (war materiel disposal service, KBD) defused the bomb just before 4pm on Wednesday.
KBD master defuser Wolfgang Wolf told the Kölnische Rundschau before the job that the so-called “block smasher” - an American weapon designed to level whole buildings with a high-pressure blast - was the biggest device he had ever worked on.
“Anyone who gets scared wouldn't be right for our job,” he said.
But disposal experts “should have respect for the size of the weapons if they want to grow old in our career. Gamblers or people who want butterflies in their stomach don't belong.”
City officials said that Cologne's largest evacuation of people since the end of the war had gone smoothly.
Roads had been closed around the city and train, ship and air traffic was cancelled.
The evacuations mostly hit the Riehl and Mülheim districts of town.
The inhabitants of a retirement home and a home for disabled people all had to be evacuated, often by ambulance. The schools in the area were closed, as was the zoo.
The bomb was found during work on an underground heating system, five meters below the surface.
“As is so common with these heavy bombs, this one is very deep down," Dieter Daeneck, who is leading the operation told FAZ.
“Such a big bomb can't simply be taken out. We need to dig a deep and wide hole first.”
Finds of this nature are not uncommon. Last year two unexploded bombs had to be defused in the same Cologne neighbourhood.