Bedrock

Bedrock is both an indestructible and unmineable block. The primary function of bedrock is to create impassable barriers, both at the bottom of The Overworld and at the top and bottom of The Nether. It can also be found at the sides of The Nether in the Xbox version.

Properties
Bedrock appears as a black and gray charcoal pattern, and emits a dark particle effect.

Bedrock is impossible to obtain without mods or Creative Mode. If placed on ground or objects, it can never be removed without using mods or Creative Mode. Bedrock blocks are not affected by pistons.

In SMP or if the "cheats" option is turned on, Bedrock blocks can be obtained by using the command, but they cannot be removed once placed without creative mode or

Survival mode
It is a common myth that bedrock can be destroyed by a sufficient quantity of TNT. While bedrock does have a finite damage resistance, there is no explosive in the game with that much force (the average block having a resistance of 15-30). Notch confirmed on the 11th of February 2011, during a Machinima Live stream, that it is indestructible, or so solid that the amount of TNT it would take to destroy it would more than certainly crash the game, therefore the save file would show no evidence of the explosion ever happening. However, later examinations of Minecraft code have shown that multiple explosions do not add damage to blocks; any block that is not destroyed by one TNT immediately adjacent to it will not be destroyed by additional TNT. Thus, the "amount of TNT" does not matter (other than indirectly by possibly crashing the game).

It is impossible to break bedrock by punching it. Bedrock has "hardness" of -1. A negative hardness is handled separately in the code, whereas damage from players' hits to the block are zero, thus the accumulated damage will always stay at 0.

As of 1.7.2, you can actually break bedrock, growing dark oak trees. The method abuses the fact that dark oak trees trees might grow downwards, if remaining parts of a previously grown tree blocks it in the upwards direction. When the tree grows downwards, some Bedrock blocks are likely to be replaced by Dark Oak Wood, which are easily removable. Bedrock does not drop. Using this method is inconsistent, and might take a few tries, but is easily done in survival Minecraft. It is mostly used to create holes in the bedrock layer on top of the nether, to make space for beacons, or simply entering the top of the nether without using Ender Pearls, as Overworld portals will not link with portals above y=128. Note that this most probably is a bug, and is likely to be patched in the future. Another way to break Bedrock, if you have cheats on, is to use the "summon" command to create a fireball entity with an explosion power of 5000. This will destroy bedrock, but is extremely rare for the item to drop, as when the explosion power is increased it is less likely to drop as an item. This will probably crash the game, but if not, the explosion will take several minutes.

In Pocket Edition, if you activate a Nether Reactor on a low enough layer, the last layer of bedrock is replaced with Netherrack, allowing access to The Void in survival mode.

Creative mode
In Creative mode, bedrock can be destroyed in one hit, just like any other block. By doing this, the Player is able to enter The Void, but will die (after falling past Y: -63), unless they are flying. This is one of the two ways to die in Creative Mode, aside from the command. Bedrock is still impossible to break via TNT in Creative mode, even with huge amounts. Bedrock can catch fire.

In the Xbox 360 Edition version of Creative Mode, Bedrock can still be destroyed, however the final layer of Bedrock cannot be broken.

In the pocket edition since 0.7.4 the void will instantly kill the player even when not fully submerged.

Location
In Survival Mode, Bedrock comprises much of the bottom-most 5 layers of an Overworld map, and both the top and bottom 4 layers of The Nether. It generates in a very rough pattern in both the Overworld and The Nether.

In the Overworld, there are actually 5 layers even though it seems as though there are 4, due to the 4th layer being predominately flat bedrock with no gaps, which makes the very bottom 5th layer inaccessible. The game code numbers the lowest layer of Bedrock as layer 0, and the highest as layer 4.

At the top of the Nether, bedrock prevents you from going past layer 127. You can use ender pearls to get through, however. After that you may use a minecart elevator or a dark oak tree to break the bedrock. Mushrooms may occasionally spawn on top of this Bedrock layer. In The End, defeating the Ender Dragon spawns a portal with a Bedrock frame. A single piece of flaming Bedrock also appears underneath Ender Crystals.

Trivia

 * The Bedrock texture is modified from Stone, at a higher contrast and placed on all sides.
 * In Minecraft: Pocket Edition, Bedrock has a second form defined in the code as Invisible Bedrock. The Invisible Bedrock is the unbreakable void-like block that surrounds all four sides of the world. This block does not use the Bedrock texture.
 * The void fog doesn't shorten visibility when sunlight makes its way to Bedrock layer.
 * Going past the Bedrock barrier into The Void will cause you to lose 2 hearts approximately every half second once you reach below Y=-63, and will quickly kill you, even if you are on Creative. However, by hovering above this point using fly mode, you are able to move around underneath the world without injury, but no blocks can be placed under layer 0.
 * In maps generated before the Halloween Update, there were occasional gaps in the bottom layer of Bedrock that the player could fall through.
 * During Classic bedrock was often referred to "Adminium" due to only admins being able to break/ place it. It is sometimes referred to by this name today.
 * As of the 1.8 Pre Release Enderman could pick up, move and place Bedrock. However, this was fixed before 1.8's eventual full release.
 * Early on in Minecraft's development there were many videos that claimed that you could break Bedrock with sufficient TNT or by mining it with a diamond pick for a very long time. All of these are fake.
 * Redstone was previously unable to be placed on Bedrock; this was changed sometime in Beta.
 * In Classic, when you walk on Bedrock, it makes no sound. Afterwards, if you walk on it for a while, and walk onto another block like stone or grass, the walking sound will rapidly play until you get off of the block or wait for a while.
 * It is possible, but rare, for ore to be found encased in Bedrock.
 * On the Xbox 360 Edition, you cannot access the void in the Overworld or the Nether, as the bottom layer of bedrock is unbreakable even in Creative Mode. However, layers above this can be broken. This means the void can only be entered in the End.
 * In Minecraft 4k, there is bedrock on the bottom of the map with no texture.
 * Strongholds can cause a bedrock not to form within its bounds.
 * However, it'll be difficult to break them due to its hard structure.
 * Bedrock, Command Blocks, End Portals, and Barriers are the only unbreakable blocks.