Bedrock

Bedrock is a dark coloured indestructible block usually found at the extreme reaches of maps. It is commonly referred to as adminium, admin blocks, adminite, and several other variations. It is a block that was first seen in Creative mode 0.0.20 as a layer surrounding the edges of a map. It later became a placeable block when Creative multiplayer was released.

Creative
In Creative Multiplayer mode, Bedrock can only be placed by Ops. It is placed by using the /solid command, and then placing Stone block. A few moments after placement (could be longer if there is lag), the stone will turn into bedrock. Non-op players cannot destroy bedrock, so it is used in making Spawn Jails, op only rooms and holding tanks for fluids. Since bedrock is not directly placed, it doesn't appear in the block selection menu. As with any blocks that aren't in the block selection menu, attempting to place one by means of hacking will cause the server to automatically kick the player. Bedrock could not be destroyed if it was in a map being played in single player. Bedrock appears a few blocks below the ocean and extends endlessly under it. It makes the edges of the map below the ocean and the bottom edge of the map. It is impossible to place blocks on the bedrock walls, but they can be built up to it by building off of existing blocks.

Indev
In Indev, if the bottom Bedrock layer of the map was exposed, it would be covered in lava after saving and reloading the map.

Infdev and Alpha
In Infdev and Alpha mode a randomly distributed barrier of bedrock exists throughout the bottom five layers of the map. It is possible, and sometimes fairly common, to find gaps in this barrier through which players can see daylight. If a player falls into one of these holes they will continue downwards and eventually start taking damage until they die. Bedrock can be given to players in Survival Multiplayer using the /give command. However it is inadvisable to do so, considering the blocks will be placed permanently, unless Map Editing is used. There is a single way to destroy it however. Bedrock placed by players can be destroyed with a lighter unlike the ones on the bottom of the map. It is unknown if this is an intended to combat griefing or a glitch.