In Germany we find bombs all the time from WW2. They have a specialized team for disarming these. And in the case they can't disarm they blow it up after evacuating from a certain distance.
I used to live in Brest which was also heavily bombed during WW2, and it's the same.
Every time they dig to start a new building, they find some exploded bombs. And each time, it's an evacuation of a lot of people in the city center to deal with it (easily a few hundred meters which in a city center means a few thousands people).
However, leftover bombs from WWII are a bit different. They tend to be bigger, hundreds of kilograms where the "typical" shell you find in old WWI battlefields are less than 10 kilograms. Also the problematic is a little different since on one side, it's a medium to big city, on the other it's mostly the countryside.
> I used to live in Brest which was also heavily bombed during WW2
Birmingham is the same. There was a WWII bomb found last year right by two major roads where they cross a railway line. Travel into the city centre was completely stuffed up for two days. I felt the controlled explosion three miles away.
