Void

The Void is the term given to the empty space beyond the generated boundaries of each map. It can be seen through gaps in the lowest layer of Bedrock on maps generated prior to the Halloween Update, but also simply by looking up from ground level, as the sky and the void are one and the same. For this reason, the void is a light source, and its brightness depends on the time. As per a recent update when lava generates on layer 2 the bedrock on layer 1 below it is replaced by smooth stone meaning it is possible to enter the void again through much larger entrances than ever before without hacking.

It is potentially endless, although falling excessively far would cause the game to crash due to memory limitations and obscenely large numbers whose digits extend beyond those capable of being processed. However, falling into the Void causes the player's health to be depleted by about 4 hearts per second, leading to the player's death in 2.5 seconds or less, should he or she drop.

In The Nether, the Void is red. It can be entered if one uses a map editor to make a hole in the top bedrock layer of the Nether.

Entities (blocks, mobs and items, excluding the player) entering the Void will be deleted in the following tick (as soon as possible). This may be due to the Void not being part of a Chunk, although it is most likely a deliberate optimisation/crash prevention method implemented by Notch. For this reason, Blocks cannot be built on the underside of the Bedrock layer, and although Water and Lava appear to flow into it, they will not actually spread and form beneath the lowest layer. This prevents players from descending into the Void safely via swimming.

There is currently a bug in SMP that will remove chunks client-side, so while the data is still there, the client cannot see it and will glitch if they step on it. This can be fixed by reconnecting, placing a torch next to the missing chunk or changing the fog distance by pressing F. If you ride a Minecart into this missing chunk, and then you step out of the Minecart, you will fall to the void and die, regardless if you disconnect and reconnect.

Light cannot travel out or through into The Void.

Destination
It is easy to get into the Void, but it is only quick if you do it by inventory hacking or taking a long time to get equipment. First, put all the items that you don't want to carry with you in a Storage Chest. Then, pick a location on the map in which you want to dig a hole straight to bedrock. It is recommended to carry a high quality pickaxe and shovel to make the destination to the bedrock take less time. This is why you should also put the items you don't want to carry in a storage chest, since mining/digging straight down is dangerous, because you might hit a cave with a long fall, or fall into lava. When you hit bedrock, mine out a bigger "cave" area to move around. Look for stone blocks which look like they are about the same level as the bedrock, and mine them. Make sure that this time you don't dig the stone straight down below you as you might fall in to the void before you are ready. Once you find a hole leading into the void which you can jump down, drop any remaining equipment that you don't want to lose, and jump down into the void. You will die in about 2 seconds making the whole effort seem totally pointless, but that doesn't matter anymore. This will only work on maps generated before the Halloween update.

TL;DR - Doesn't work anymore.

There is also an alternate way (one that actually works), which involves much less searching. First, get a boat. This is easy enough, considering it involves punching all of three logs for the resources. Dig down to bedrock, and find the lowest point you can (F3 should show about 2.62 for the Y coordinate - this is actually 1 because the reading is from your eyes). Put the boat down on the ground, get in it, then destroy it while inside it. You should fall through the bedrock, and end up in the Void. You'll still die in two seconds, but hey - now you can say you entered the Void through bedrock. The same effect can be achieved with a minecart placed on a rail (minecart track), entering that minecart and then destroying it.

Media
Click here for A video of The Void when fallen into.

Click here for A video of The Nether's Void.

Click here for A video of an entrance to the Void caused by lava.

Trivia

 * If you open a hole to the void and block off all light sources, you can see that the void does in fact give off light. However, the bottom of the bedrock on the last layer is completely dark.
 * When you fall through the void, you don't get hurt until you reach the Y-coordinate of -62, and then you get hurt for two hearts per second.

Пустота