The Bedrock Block is a naturally occurring block that appears in both Normal Worlds and Flat Worlds, but only at the farthest depth in the World. It is not placeable or even destructible, and can only be removed by either hacking or buying a mod pack off the internet (which in itself is a hack). People also create underground cities, using the bedrock as a floor or street. Some Edeners have even cleared out a big space of bedrock and built a skyscraper from bedrock to the height limit.
In normal worlds, there was one never ending layer of bedrock, and one layer that mixed with stone.
In flat worlds, there is only one layer of bedrock.
TNT cannot destroy this Block.
Before Caves were removed, bedrock would be somewhat jagged instead of being flat.
As of now, bedrock is the only block immune to TNT, but in 2.0, Steel will also be one of the two immune-to-TNT blocks.