TNT is a by nature explosive block.
This block was used for decoration, but in the 01.06.2011 nightly, TNT was given the ability to explode. You can interact with blocks and ignite TNT with 'E', if no other key is set and your version of Manic Digger supports it.
TNT currently can't hurt monsters or the player.
You can craft TNT with 1 Gold Block.
|Gold Block, 1||TNT, 1|