The Void

The Void is an area/dimension in Minecraft Pocket Edition found under the layer of Bedrock. It was the only way to die in Creative Mode, but since Update 0.11.0, players who fall into the void in Creative Mode will not die as an invisible barrier was erected at y= -60 to keep the player close to the Overworld.

Creative Mode
On Creative Mode, players must first reach the layer of Bedrock and then dig through it. They will fall into the Void and keep falling till they started flying or fell onto Invisible Bedrock. Before Update 0.11.0 they would die instantly.

Survival Mode
On Survival Mode, there were two ways to access the Void (one way as of 0.12.1). One way is by activating the Nether Reactor (no longer possible as of 0.12.1 as the Nether Reactor and its functions were removed in that version). The player must build the Reactor on the bottom layer of Bedrock and activate it. The Bedrock will get replaced with Netherrack and the player will be able to break it and access the Void. The second way is to fall into the Void is to break the Bedrock layer in Creative Mode, then switch to Survival Mode. On Survival Mode, the player will die, and a death message-  fell from a high place will be shown, unless the player jumps into the Void with low health (in which the player would die from Void damage) and the death message  fell out of the world will be shown.

Trivia

 * Players will not lose their items if they die through Void damage in Survival Mode.
 * Mobs will die too if they fall into the Void.
 * In other versions, the Void cannot be accessed.
 * In Superflat Worlds, you cannot see the Bedrock layer at the Invisible Bedrock. But if your render distance is high enough, in other world types the bedrock is visible.
 * Items will disappear once they hit the Void.
 * If the player generates a world on a version of Minecraft which only has Old Worlds, and then upgrades the game to one with Infinite Worlds, and then converting the Old World to an Infinite World, there would be large square pits outside the Old World boundaries that lead to the Void.