In WW1, between 900 million and 1,2 billion artillery shells were fired. That is just artillery shells, not other projectiles which may contain explosives, hand grenades etc. On very very limited areas of the world. Its estimated around 300 million projectiles (not just artillery shells) were duds. Its not like people missed 300 million of them, but when you got 300 million of various different kind of explosives here and there, you will miss some. Remember that artillery would keep firing after those duds had landed, so ground would keep changing and those might get buried deep, only to be discovered much later on by farmers who touch the stuff, bringing ground from the bottom closer to the top.
And then you got second world war, which saw massive scale aerial bombardments as well. Those bombs would scatter over large area, they were very inaccurate. Idk how many of them exploded or were dropped, but its good amount.
Its not like people were stupid
[https://en.wikipedia.org/wiki/Iron_harvest](https://en.wikipedia.org/wiki/iron_harvest)
If you were to scatter exactly 1 million coffee beans around your apartment, it would propably be very difficult to find ALL of them. And your apartment does not hopefully have a ground which is constantly changing and shaped by artillery bombardment, burying coffee beans deeper.
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