Mines can only be placed by a battleship. It takes the whole turn for the battleship to place the mines. As a consequence, the battleship cannot move and place mines in the same turn, it must place mines where it starts its turn. Place a marker of the country who laid the mine on the territory mined. When enemy ships enter a mined territory, each ship must roll a dice, and sinks on a 1.
Subs are immune to mines.
A player can decide to communicate the position of the mines to an allied player, in which case the allied player can enter the territory without rolling. A battleship can also remove ennemy mines. We implemented the events with cards, but you can just print them out as a list and randomly draw them. The events are divided in 2 stacks. One that helps the Allies, one that helps the Axis.