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 are not required, and only serve for aesthetic purposes. Once the frame is constructed, the player then sets the space inside the frame on fire, using a flint and steel. 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. The player can step out of a portal before it completes its animation 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. Building multiple portals on Earth within a certain proximity will all lead to the same portal in The Nether, and the same might happen vice versa.

The purple portal blocks emit light, so passive mobs may crowd around active portals at night. Like Bedrock, portal blocks cannot be broken (although they can disappear); however, they can be placed with modding. These portal blocks will automatically disappear after a few seconds, or when a block is placed adjacent to 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. However, portal blocks will not disappear in SMP.

Portals can deactivate if there is fire adjacent to it (started by the player or Ghasts). TNT can also disable a portal, but cannot destroy the surrounding Obsidian. It is possible to "re-ignite" portals by setting the space inside the frame on fire once again.

Contrary to public belief, portals do not conserve momentum.

It has been confirmed that portals do not work in multiplayer yet; while they can be created, they can not teleport you. Server settings can be changed in order to visit The Nether in multiplayer.

Notch has mentioned it is extremely unlikely, but possible, for Ghasts to appear on Earth near a portal.

Portal Bug
There is currently a flaw in the mechanic where during the creation of a portal in the Main world, the game automatically locates a portal in a relative location in The Nether. However, there is a chance that the place where the portal should be situated is already occupied with a solid block or lava, causing the game to move the portal to another location. This causes the player to 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).

It is possible to make a portal you build on the overworld go both ways with a portal in The Nether. To do so, a new portal must be built 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 on 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.

An Obsidian farm can be created with this bug by mining the secondary portal and then traveling through the primary portal again, repeating until the desired amount of Obsidian is obtained.

Trivia

 * 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.
 * If you run out of Flint and Steel and disable all of your portals while in The Nether, it is still possible (although difficult) to reactivate a portal by having a Ghast's fireball hit the portal instead of you.
 * When you look through an activated portal with water behind it, the water will not be visible.
 * Likewise, if you look through a portal with water in front of it, the portal blocks will be invisible.
 * Even when a portal is built with only 10 blocks of Obsidian (by leaving out the corners), the portal frame spawned on the other side will have the full 14 blocks.
 * If a portal created in The Nether would exit into midair on Earth, the overworld side will have four additional blocks creating a larger ledge to stand on.
 * It may be a glitch, but the player can strike portal blocks with tools, fists, and blocks, but objects will pass through them. This also explains why you can't put blocks and fire inside the portal frame while activated.
 * WARNING! Riding a minecart into a portal has some adverse effects. The player can go through the portal, but remains stuck in the ceilling of The Nether. Reloading that saved world again will drop the player into the Nether, possibly without a matching Nether-portal and possibly right into a lava lake. Riding a pig or a boat through a portal also gives this same effect. The object you rode will stay on the Earth side of the portal.
 * Trees, cacti, and wheat are all plant-able in the Nether, but water needs to be hacked in to grow reeds. Planting these have the same system as the overworld.

Media

 * Here and here are two preparation videos with commentary, in which people craft portals according to Notch's specifications.
 * On 10 October 2010, Notch revealed the above screenshot on his blog showing portals.
 * On 29 October 2010 PC Gamer released this video, showing a portal being constructed and used.