The dig tool lowers land.
Brush: The size of the brush affects the number of pixels that are affected when the tool is activated.
Strength: Determines how much the brush digs the land when the tool is activated.
Type: changes how the land is dug, whether smoothly, pointed or square topped. Alternatively, you may specify your own custom brush from the paint brush settings dialog
Fixed Height: locks the tool to a certain height level and then performs the brush action.