Another way to do it would be to place a trigger, make it the size you want,
for the purpose of this ex lets say its 5 by 5 meters.
use the Anybody present, one only, and call the trigger something, lets say mine1.
then in the on activation field, place
Bomb="Shell75" camcreate(getpos mine1)
Shell75 could be shell120, LaserGuidedBomb, and so one, depending on how much power you want.
increase the size or witdh of trigger as desired, and if you want something on the ground to represent the mine, you can just place a standard mine, and put in right in the middle of the trigger.
I just remembered, you cannot use the anybody present condition, if you place something inside the trigger, so if you do place a mine in there, you will have to have it on East, West, so on.