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. Once the frame is constructed, the player then sets the space inside the frame on Fire. This creates a swirly purple vortex inside the frame. When the player stands in the purple vortex for a few seconds, the player is taken to The Nether. Stepping into the portal (purple blocks) will begin a teleport sequence which, after a few seconds, will transport you to the other side. You can step out of the portal before it completes to abort the teleport.

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.

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 = 16 in 'earth'. Build a new portal in The Nether at the appropriate location, and your original portal should now link to it both ways (reference thread).



The portal can also be built without the corners.

If spawned through modding, portal blocks will automatically disappear after a few seconds, or when a block is placed adjacent do it.

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.

When you look through an activated portal where there's water behind it the water will appear as if it's not there.

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. This again references that The Nether is below  the 'earth'.
 * It has been reported that building multiple portals on 'earth' within a certain proximity will all lead to the same portal in  The Nether.

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.