Create a trigger, set its radius to 1 or 2 metres, set activation to "anybody", "present"
Put this in code in the activation field:
MinePos = getpos (thislist select 0); explosion = "AA" camCreate [(MinePos select 0),(MinePos select 1), (-7.8 + (random 1.5))]
This creates an explosion(with crater) that will either kill or wound a soldier who goes near. Also damages other vehicles.
You can copy and paste the triggers to cover the whole island with mines if you want. There is no limit to the number you can place, I think, and there is very little lag when you place hundreds of mines. Try it!