The Void is the empty space under each map. It can be seen through gaps in the lowest layer of Bedrock.
This space is normally light blue in colour, since it is always daytime in the Void. Falling into the Void will cause the player's health to rapidly decrease, eventually killing him. After respawning, the player will have lost all held items with no way to recover them. Items, falling Sand and Gravel, and mobs will disappear if they fall into the Void. Water and Lava will appear to flow though it, though they won't appear under the map.
The Void, found on the bottom of the map.