During WWI, it was so muddy that many shells penetrated very deep into the mud, and remain there today, gradually incing their way to the surface. On a visit to Belgium a few years ago, we were told that even 100 years after the end of WW1, they estimate it will take another 100 years for all of the unexploded WW1 ordinance to finally make its way to the surface. The "Iron Harvest" is an interesting thing to see. And that's just farmers fields.
There has been a fairly concerted cleanup for the better part of the past century (WW2 years excepted), but there is a long way still to go.