Nether portal



The Portal is part of the October 31st Halloween Update. It consists of a frame of Obsidian, four blocks wide, by five blocks tall. The corners of the frame do not need to be made of obsidian. Once the frame is constructed, the player then sets the space inside the frame on Fire. This creates 6 portal blocks inside the frame, resembling a vortex. Here is a diagram of the process:



When the player stands in a portal block for a few seconds, the player is taken to The Nether. You can step out of the portal before it completes to abort the teleport. A portal will also be created in the place where you enter The Nether, which you can again enter to be returned to the normal world.

Contrary to public belief, portals do not conserve momentum.

The purple portal blocks emit light, so you may find passive mobs crowding around active portals at night-time.

Like Bedrock, portal blocks cannot be broken (although they can disappear), However they can be placed with modding.

When a portal is first created from the overworld to the Nether, there is no guarantee that using the reverse portal will send you back to the original portal. The reason for this is unknown, but is possibly related to placement issues (the Nether portal might have to be moved because it would otherwise be in solid rock/lava/etc.)

It is possible to make the portal you build on 'earth' go both ways with a portal in The Nether. To do so you need to build a new portal in The Nether in a position relative to where the portal you first built is. When you travel to The Nether, and then return, you usually emerge from a different new portal somewhere near your original one. Work out the relative position of the new portal and your original, and remember that travel in hell is around 1 block = 8 blocks in 'earth'. This means that, at some locations on 'earth', that a new portal will NOT be created, as the original one is in the "correct" spot. Build a new portal in The Nether at the appropriate location, and your original portal should now link to it both ways (reference thread).

While they can be created in multiplayer, the portals will not teleport you.

Though obsidian will never be destroyed by TNT, it can disable the portal so that it needs to be "reignited".

Portals can deactivate if there is fire adjacent to it (started by the  player or Ghasts). It is possible to activate it again by setting the  space inside the frame on fire.

It has been confirmed that portals don't yet work in multiplayer, though you can change a server setting in order to visit The Nether in multiplayer. (See this forum post.)

Trivia

 * It is interesting to note that if you place two portals on top of one another you will come out of the bottom one on 'earth' and the top one  in The Nether.
 * It has been reported that building multiple portals on 'earth' within a certain proximity will all lead to the same portal in  The Nether, and vice versa.
 * If a person somehow runs out of flint and steel and disables all of his portals, it is still possible to relight the portal by having a ghast fire at you and hit the portal instead of you. though this is difficult, it is possible.
 * When you look through an activated portal where there's water behind it the water will appear as if it's not there.
 * If there is no room on one side of the portal you built in the real world (i.e. against the side of a mountain), you might not return to it when using the same portal on the Nether side. Instead, you will arrive at a newly spawned secondary portal in the real world, in the same direction but some distance from the original portal, where there is sufficient room (i.e. within a cave).
 * If spawned through modding, portal blocks will automatically disappear    after a few seconds, or when a block is placed adjacent do it.  The  only   exception is to place an obsidian block, and a portal piece next  to   that.  The portal piece will disappear once the obsidian block is  gone.

Media
Here are some preparation videos and commentaries in which people craft portals according to Notch's specifications. here and here.

On 10/10/10 Notch revealed the above screenshot on his blog showing portals.

On 29 October 2010 PCGamer released this video, showing a portal being constructed and used.