Minecraft wiki entry for Mining, associated text: In TNT mining, a tunnel is created around y=15. Once the player has gone far enough, or ran out of TNT, they will go back along the tunnel, placing TNT on the floor every 5 blocks (that is, with 4 blocks between each one), until they get to the beginning, where they will light up the TNT, causing a chain reaction. In the resulting tunnel, there should be exposed ancient debris left intact by the explosions, that can be mined out.