The Void

The Void is the area below bedrock, only accessible in Creative Mode or from the Nether Reactor. Users will die if they fall too far into it. It is the only way to die in Creative Mode.

Accessing
In survival you must take many steps to reach the void. You must use the Nether Reactor to access the void due to the fact that it changes the floor and layer below it into Netherrack. Dig down until you hit bedrock, then make yourself a flat space with sufficient room to build the Machine on. Then activate it. You may need to do this up to three times to dig past enough bedrock.

In Creative getting it fairly easy and straight forward. First you dig a hole to reach Bedrock. Since you are in creative you can easily dig through the bedrock and access the void. Although, you can't place blocks below the lowest level of bedrock. If the player goes any lower, he/she will die. In the older updates, you could go 3 or 4 blocks below the bedrock and travel around the Void, unless you went too low, which would result in death.

Trivia

 * In Creative, it is white, while in Survival it is black.