TNT has a blast radius damage of 4. So we divide 18M by 4. Which would equal out to: 4,500,000. Meaning, it would take 4 million, 500 thousand TNT to destroy said block. And since blocks regenerate health when something Isn't touching it, means that you'd have to set of all 4.5M at ONCE. And I'm fairly sure no computer in the world could handle that.
I'm not sure if you can actually mine bedrock. Code for bedrock. hardness determines how hard it is to mine with a pickaxe, and resistance is how hard it is to destroy with an explosion. So, I'm pretty sure it's impossible to mine -1.
bedrock = (new Block(7, 17, Material.rock)).setHardness(-1F).setResistance(6000000F).setStepSound(soundStoneFootstep).setBlockName("bedrock");