Bedrock

Bedrock, otherwise known as Adminium or other variations, is an indestructible, unmineable block. The primary function of bedrock is to create an impassable layer at the bottom of the Overworld. In the Nether, it creates a barrier all around the map, and as in the overworld is indestructible.

Survival
Bedrock is impossible to obtain without mods or creative mode. If placed, it can never be removed without using mods or Creative Mode.

In Survival Multiplayer, Bedrock Blocks can be obtained by using the /give command. It is not advisable to do this because the Blocks will still be impossible to destroy when placed without a third-party server plugin or the /gamemode 1 command.

Post-Classic, there is no /solid command. The only way to place Bedrock on an unmodified server is /give.

Destructibility in Survival
It is a common myth that Bedrock can be destroyed by a sufficient quantity of TNT. Examinations of the 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. While Bedrock does have a finite damage resistance, there is no explosive in the game which has 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.

It is impossible to break Bedrock by hitting it. Bedrock has "hardness" of -1. A negative hardness is handled separately in the code, and causes any damage done to the block by the player's "hits" to be zero, and thus the accumulated damage will always stay at 0.

Creative
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 still die, the only way to die in Creative Mode. Bedrock still has high blast resistance and you cannot break it with TNT. Bedrock can also catch on fire and burn if hit by a fireball, it is unknown if this is a bug or glitch.

Classic


In Singleplayer Survival Mode, Bedrock is impossible to place without a mod. This is because Bedrock is never directly placed, but rather by using a server command to change other blocks into it if it is on multiplayer. Any Bedrock that exists on the map cannot be destroyed by the player.

In Multiplayer mode, server ops can place Bedrock blocks by using the "/solid" command. When /solid is used, stone blocks turn to Bedrock a short time after being placed. If /solid is used again, stone blocks placed will remain stone blocks. Normal players cannot place Bedrock; this makes Bedrock useful for making spawn jails. However, ops can destroy Bedrock like any other block, as a flag is set on the client when they log in. Vats of fluid blocks (water and lava) are also recommended to have Bedrock barriers so as to prevent server floods. Bedrock is used as a dark colored building material for normal structures as well.

A player can use an inventory editor to put Bedrock blocks in their inventory. If they attempt to place the blocks, the server will automatically kick them, even if they are an op. This is one of Notch's anti-griefing features scripted into server software.

In all Classic maps, the very edges of a map are lined with a Bedrock-textured barrier. This barrier is not actually made of Bedrock blocks; it is simply a flat, impassible wall with the Bedrock texture. Consequently, attempting to place blocks on it does not work; you must place a block on one already attached to the wall. This barrier appears on the sides of a map starting two blocks below ocean level, extends along under the ocean and across the entire bottom of the map. Additional walls made of Bedrock blocks are often built against the ocean to prevent flooding in custom servers.

Post-Classic (Indev-present)
In Survival Mode, Bedrock appears on the very bottom 5 layers of a map and also at 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 if there are 4, this is because the lowest one you see is normally entirely Bedrock with little or no gaps. The lowest layer of Bedrock is layer 0, the highest is layer 4. It can also be seen at the top of the Nether, preventing you from going past layer 127. As of 1.9 Pre-Release 6 when you defeat the Ender Dragon a portal with a Bedrock frame appears. A single piece of Bedrock will appear after destroying an Ender Crystal.

Layout and position
Bedrock can be found at the bottom of the map in five different layers. Holes in the Bedrock can be found leading to the void. Just like in any mode if the player enters the Void he/she will die. The Nether also has a Bedrock bottom as well as a Bedrock top (post-1.2 the "top" is now only in the middle due to the doubled map height). On occasion, mushrooms will spawn on top of Bedrock in the Nether. Bedrock can be found in the End, but not at the bottom of the world, instead at the top of the obsidian pillars and as part of the exit portal after the Ender Dragon dies.

Void fog


Void fog was added to the beta 1.8 update so that The Void would appear black instead of sky color. This black fog (with gray particles floating in it) thickens as the player reaches Bedrock levels, regardless of render distance settings. See the graph (right) for the maximum view distance due to the fog.

Void fog can be driven away by exposing a block(s) to the sky. This will create a small area on and around the exposed blocks, which is free of void fog. Exposing blocks in this case means that a block can have nothing but transparent blocks above it all the way to the top of the map (layer 255).

As of Minecraft 1.3 Void fog start at layer 26, anything above that will be 100% free of void fog. Any unexposed blocks below layer 26 will have void fog, getting worse the lower you go.

Outdated Versions

 * Classic - Unbreakable.
 * Survival Test - Does not spawn in the map, cannot be destroyed by explosions, but it is breakable and drops a normal resource. However, it takes more than 16 minutes to break a single block.
 * Indev - Unbreakable, but Bedrock on the bottom of the map will turn to Lava after a while if it is exposed to sunlight.
 * Infdev, Alpha and Beta (pre-Creative Mode) - Unbreakable, no exceptions.

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're on Peaceful.
 * In maps generated before the Halloween Update, there were occasional gaps in the bottom layer of Bedrock that the player could fall through.
 * Since the addition of lakes (of lava and water) at any height, it is possible for a lake to cause an unusually large gap in the Bedrock.
 * In versions prior to Beta 1.3, Bedrock could be destroyed by planting a seed below it on tilled dirt.
 * There are a total of 5 layers of Bedrock at the bottom of a world, from Y=0 to Y=4.
 * Prior to Beta 1.6.0, if you placed a boat or minecart tracks with a minecart on the lowest Bedrock block, then climbed in and destroyed the boat/minecart, you could fall into the void.
 * It is possible to get stuck in Bedrock layer if you don't have any blocks to help yourself get out.
 * Notch tweeted he had accidentally done something to the Bedrock layer, but didn't specify what. It appears that close to Bedrock, the fog levels become closer, and there is an ambient particle effect at this level, similar to the Void.
 * As of Beta 1.9 pre-release 6, you can Turn off/lower the particle effect.
 * There is a very, very rare possibility that a dungeon can be generated in the Bedrock layer, which erases the Bedrock wall, and you can easily fall into the void by mining moss stone or cobblestone in the floor of the dungeon.
 * In Indev, the Bedrock layer was smooth.
 * There is also a very rare possibility that an abandoned mineshaft can generate at Bedrock level, causing the Bedrock to be removed. Note that this no longer works as of 1.2.4, because abandoned mineshafts will now generate with a "bridge" of wooden planks under them instead of being floating. There is, therefore, no way to have an open hole to the void.
 * Redstone was previously unable to be placed on Bedrock; this was changed sometime in Beta.
 * As of 1.0.0, Bedrock can be found by 5 different means: at the bottom of the Overworld, at the bottom of the nether, at the top of the nether, when an Ender Crystal is destroyed, or when the Ender Portal in The End spawns after the dragon is defeated.
 * In Minecraft classic, when you walk on Bedrock, it makes no sound. After 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 you wait for a while.
 * It is possible, but rare, for ore to be found encased in Bedrock.
 * In an 1.2.5 Snapshot, if the player has no Bedrock in their hotbar while in Creative Mode, pressing the "Pick Block" button while looking at a mob will give that player Bedrock.
 * As of 1.2.5, Bedrock in the End will burn forever if set on fire, similarly to Netherrack. It can still be put out by the same methods, as well.
 * Bedrock blocks are not affected by pistons.
 * Mobs can not spawn on Bedrock as of sometime in version 1.2, this was implemented because of the new block height level, mobs were spawning on top of the ceiling layer of bedrock in the Nether.