there's a lot of it about. We sometimes hear sea mines being blown up the Royal Navy, or hear about them being caught in fisherman's nets. Must be way worse in Germany.
Not only that, but they're still finding chemical munitions from WW1 around europe. There was a documentary on the BBC last year called "Inside Porton Down" where they showed the lengths they have to go to disarming and destroying these compounds. Despite being 100 years old they still maintain their potency and could kill or maim easily if handled incorrectly.
There was a minefield laid between Scotland and Norway in WW1 and then a similar one laid in WW2. There are potentially over 100,000 devices still out there from what I remember reading.
We also dumped an awful lot of nasty stuff in Beaufort's Dyke, a trench in the Irish Sea.
Another reply mentioned the Porton Down TV documentary, that is well worth watching.
Very common in Italy. My hometown has always been a critical railway node, so it was heavily bombed by USAF; any time they start building on unused land, they inevitably find some unexploded ordinance. It's super-weird when they just take down a building, re-dig the site, and lo, they find stuff underneath that could have exploded at any time in the last 75 years killing hundreds...
Germany here: there's news about parts of some city or other being temporarily evacuated every couple of months. Usually the bomb is defused without incident.
Here's one example where they had to do a controlled detonation on site: https://www.youtube.com/watch?v=_NbM2Xbc1uk - not so controlled after all, since the straw bales they used to dampen the shockwave cased several fires.
Most of the North Sea and Baltic Sea are have been cleared but some areas remain full of mines. And there's no real desire to clean those up if it's in areas shipping/fishing doesn't use. Makes it harder for other nations to hide submarines in case of a future conflict.
And, in the most heavily-impacted areas such as Verdun, they estimate that it will take not less than 700 years to clear the areas, at the present rate.
