Minecraft wiki entry for TNT_cannons, associated text: This cannon requires water, redstone, any block, TNT, and pressure plates. Make a 9x3 rectangle, break the middle of a 3 on one side, and put 2 more blocks on both sides of the hole, straight up on the blocks. Pour water into the other side, and place TNT everywhere but source, and between the highest blocks. Make stairs down one side of the high areas, place redstone on the stairs, one block away, then add a pressure plate. Place redstone along the frame until you get to the high points. Put three redstone past the far corner by the stairs, and place another pressure plate. Step on that, wait a count of 3 or 4 secs, then go step on the other plate. The high TNT will fly out up to about 50-60 blocks.