A mine increases the shield production of a square. Terrain Produces Grasslands 0 shields, 1 if mined. w/ outcropping 1 shields, 2 if mined. Plains 1 shields, 2 if mined. Hills 1 shields, 3 if mined. Mountains 1 shields, 3 if mined. Workers are ordered to dig mines by pressing M, or the "Build Mine" button.