Bedrock

Bedrock, also known as Adminium or Hard Rock, is a block that is impossible to break when in survival mode whose purpose is to completely prevent passage through any means, in Survival Mode. Bedrock is the barrier to and is right above The Void and its layer can be 1 to 4 blocks high on the Y-Plane. An uncovered layer of Bedrock is not flat; notice the uneven placements

Description
Bedrock is a block with a gray and charcoal pattern found at the bottom of all Minecraft Maps and at the top, bottom, and a few sides of the Nether as well (Xbox Edition only). It exists in randomly generated layers, creating a usually-impassable barrier that prevents players from falling past the bottom of the map. If the player is able to wriggle past this layer for some reason (or destroy it using Creative mode / hacking tools), the player will fall into The Void and die after falling past Y=-63, although you can find your way out if you quickly turn on Fly-mode or noclip and fly straight up. If you are too slow, you will never find your way back out, for the square of land you came in through will turn as black as the rest of The Void. In a Superflat world, bedrock can be found after digging down three blocks of dirt. (According to updates after 1.4 when Superflat worlds were editable, it now varies of what map you seed up).

Bedrock is effectively, but not completely, indestructible. In Creative Mode, the player can create and destroy it as easily as any other block. In addition, game hacks can also remove Bedrock, as can server administrators. As far as Explosions are concerned, Bedrock has the maximum possible blast resistance value of 18,000,000, which means that it would take the explosive force of 4,500,000 blocks of TNT to destroy Bedrock. Such an explosion would also destroy a large section of the map and crash the game stating that java has run out of memory.

Bedrock can also be found in The End. On top of some of the Obsidian towers in The End, there is a block of Bedrock with an Ender Crystal floating above it, which is used to heal the Enderdragon.

Furthermore, Bedrock cannot be broken by any means in survival. This is due to the hardness being -1, causing the block to take no damage regardless of the level of efficiency on a pickaxe or the time taken to mine the block.

Trivia
An uncovered layer of Bedrock is not flat; notice the uneven placements
 * The Pirate Speak language setting calls Bedrock "The Har'est Rock".
 * The particles that appear to be emitted from Bedrock are actually emitted from the Void, since the particles go through the bottom Bedrock layer. Bedrock created at a level much higher than the Void don't emit particles.
 * The bedrock can be found in layers 0-4. (It is higher in Superflat)
 * Bedrock is also used as a spawn jail, since it's unbreakable. It's used as punishment for griefers after removing the creative powers from them.
 * In Minecraft Classic, Bedrock can always be found at the end of a map.
 * Bedrock can catch on fire.
 * TNT minecarts will explode if they hit bedrock!