Minecraft wiki entry for Mining, associated text: When you reach the designated y-level, first make sure where you want the floor of your mine to be. Now count 3 blocks up from the floor, and place a sign on the wall in the column with air (i.e. below the first sign). Then, place the third sign next to it on the wall, right in the middle of the water column. Now the water will be blocked, and will flow sideways over the second sign. When you are finished, you should have 3 blocks of clearance between the floor and the water, and you can see water above both signs. Now you are basically done with the shaft, and can expand your mine however you like.