First, create a new unit called 'Mine Layer/Infantry Transport' or whatever. Give it the Radar Artillery graphic. Give the unit move-2 attack-0 defense-0, Carry 2, Carry Foot Units Only, requires the tech that tanks require (I forget). Creat the land mine unit, give it move-1, attack0, defense-10, bombard attack-30, bombard range-0, bombard rate of fire-3, Foot Soldier, Immobile, Hidden Nationality, requires same tech as the mine laying unit. The mine laying unit doubles as an infantry transport, and the mine will damage units as they attack a tile, but will be less effective against more modern units, simulating their increased ability to detect mines. Any of the numbers except the zeros can be changed for balance. Also, you'll need to create a capture animation for the mine layer.