There are pre-agreed patterns to which landmines should be laid. The areas should also be marked and mapped to facilitate cleanup. However, this does not occur!
The most effective way to sow the mines is through air-drop. Guerillas and rebels do not follow patterns, and instead choose to plant mines where they are most likely to be found (as the saying goes, curiosity kills the cat!). No one keeps note of where the mines are!
As such, land mines have become the single largest threat to the health of rural populations in countries that once suffered warfare.
Some countries, like Bosnia, Iraq and Kuwait, are heavily mined but stand a chance of being cleaned up. However, some others have actually become dumping grounds for millions of cheap mines laid down to terrorize the population, these include many African countries, Afghanistan and Cambodia.