YOU DON'T HAVE TO BE THE ONE WHO MINES THE COBBLESTONE! As long as you place 27 stacks of cobblestone into a chest, you will get the achievement. This means that if you get 2 or more people strip mining, you can get 1728 cobblestone in no time. Give all the cobblestone to one person, then they put all the cobblestone in a chest. Then the other person takes all the cobblestone out and puts it back in. Repeat for all players.