France still has WWI unexploded ordnance, and keep-out areas are still being de-mined. This has been going on for a century now. About 900 tons of explosives are removed each year. Completion in 700 years at the current rate.[1]

[1] https://www.warhistoryonline.com/world-war-i/the-red-zone-la...

That is mind blowing, no pun intended.