Bedrock

is the block that makes up the very bottom of the world, preventing access to the Void. It has a 18,000,000 blast resistance, making it the hardest material in the game. It can only be destroyed in Creative Mode. When attempting to mine bedrock in Survival Mode, it will take health off of your pickaxe and destroy it within 20 minutes. The bedrock is 5 layers deep.

Trivia

 * Bedrock can be broken in Survival mode if you make a large enough space and activate the Nether Reactor on the last bedrock layer, after the tower is complete, mine under the tower and you will find the the gap where the Bedrock was broken, giving access to the void.