Bedrock

Bedrock is the block that makes up the very bottom of the world, preventing access to the Void. It has an 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 durability off of your pickaxe and destroy it within 20 minutes (This no longer occurs as of 0.6.1). The bedrock layer is not flat, but is usually around 4-6 layers deep. Underneath bedrock is the Void, A dark and mysterious endless hole called "the end of the world". If you fall into it, you will die. This is the only way you can die in creative mode.

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 (generated by the Nether Reactor) is done generating, mine under it and you will find the the gap where the Netherrack has broken into the bedrock, giving access to the Void. You may have to activate more than once to do this, which can be expensive unless you have someone on the map to help you.
 * Invisible Bedrock makes up the sides of the world, preventing players from falling off the edge of the world. It is possible to place Ladders, Fences, Torches etc on it.
 * It is possible to go OVER the wall if you fly high enough in creative.
 * In Minecraft: Xbox 360 edition it is not possible to break bedrock that blocks the void, although you can break bedrock that you have placed.

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