[Patched in latests update] This used no mods or command blocks. Found a bug in the game where you could duplicate TNT with arrows xD
This is not just your normal blow up dirt and stone TNT, it also breaks through obsidian and bedrock! All you need is 4 TNT and a few stacks of arrows and you are good to go. This worked in vanilla survival Minecraft 1.7 & 1.8 snapshots.