[Huebolt]

The cost of producing and laying a mine is usually anywhere from 0.5% to 10% of the cost of removing it, and it can take up to 200 times as long to clear a minefield as to lay it.

Parts of some World War II naval minefields still exist, because they are too extensive and expensive to clear; some of these mines might remain dangerous for hundreds of years (Wiki, unfortunately...)

[PghBaldy]

They are still finding unexploed bombs in Europe. IIRC, even WWI ones fairly recently (within 20 years is recent to me).