This. If we assume we're talking about the order of 10,000,000 tons, of which 10% didn't detonate (this is roughly accurate for bombs & shells, mines are much h

This. If we assume we're talking about the order of 10,000,000 tons, of which 10% didn't detonate (this is roughly accurate for bombs & shells, mines are much harder to quantify), then that left 1,000,000 tons of UXO. If we assume (somehow!) half of that was dealt with quickly, that leaves 500,000 tons of UXO. If we assume 1000 tons are found per year, that's still 500 years to tidy up. Across both wars the total may well be closer to 100M than 10M, so then that makes it 5000 years.