Nether portal

A  Nether Portal is a manufactured structure which acts as a gateway between the Overworld and Nether dimensions. It is built as a frame of Obsidian, four blocks wide, by five blocks tall, after which the inside of the frame is ignited to activate it. The corners of the frame are not required, but portals created by the game will always include them.

Behavior


Once the frame is constructed, the player then sets the space inside the frame on fire, using a flint and steel, fire charge or Lava. This creates 6 portal blocks inside the frame, resembling a vortex. (The portal blocks will replace the fire.)  While a Nether portal frame can be built in the End, it cannot be lit/activated. Note: if the frame was built incorrectly, e.g. with the wrong dimensions, the blocks will simply ignite, but no portal will form.

Nether portal blocks make distinctive sounds, and emit purple particles resembling snowflakes (the same purple particles are produced by Endermen and Ender Chests). Portal blocks normally occur in a group of six filling the frame, but if a single portal block is hacked in and placed on the ground, it can still be used to travel to the Nether. It only takes one block of portal to teleport to The Nether. When a non-portal, non-obsidian block is set down next to it, such an isolated portal block will remove itself. The ambient portal music can still be heard from these blocks, even without an obsidian frame.

When a player in the Overworld or the Nether player stands in a Nether portal block for 4 seconds, the player is taken to the other dimension. The player can step out of a portal before it completes its animation to abort the teleport. (In creative mode, there is no wait time -- the player will immediately transfer between dimensions.) If there is already an active portal within range (about 128 blocks) in the other world, the player will appear in that portal. Otherwise, a portal will be created at or near the corresponding coordinates. If a portal is deactivated, and a matching in the other dimension is used before it is re-activated, a new portal may be created (not if there is another, active, portal within range). The usual case for this is a when the player's Nether-side portal is deactivated by a Ghast, and they then die in the Nether and re-enter. However, multiple portals can be exploited to farm obsidian.

Since the game looks for an existing portal, multiple portals built too close to each other will all lead to the same portal in the other world. Because of the coordinate conversion (see below), the definition of "too close" is very different between the dimensions: Portals in the Nether must be within 16 blocks of each other to reach the same portal in the overworld, but portals in the Overworld will reach the same portal in the Nether if they are within 1024 blocks of each other.

The purple portal blocks emit light of level 11, rather dimmer than a torch. Like Bedrock, portal blocks cannot be broken by tools; however, they are destroyed by even weak explosions. TNT, a Ghast's fireball explosion, an exploding Bed, or an exploding Creeper can all disable a portal, but cannot destroy the surrounding Obsidian. A bucket of water dropped inside the frame of an active portal will also deactivate it. It is possible to "re-ignite" portals by setting the space inside the frame on fire again, or with another explosion. However, a fire lit next to a portal cannot deactivate it.

As of snapshot 12w34a (version 1.4), entities can travel through portals. This includes mobs, thrown items, and even empty boats and minecarts. Storage Minecarts and Powered Minecarts can pass through, but minecarts or boats with passengers (mob or player) cannot. Thus, interdimensional railways are limited to cargo (and chunk-loading issues limit even that use -- see below). Note that mobs have a much longer "cooldown" time than the player, so they can't go back for 30 seconds or so, by which time they will have wandered or been led away from the portal.

If you inset your portal frame into the floor (that is, the portal proper is level with the floor), that will make it easier to shove mobs through the portal. Besides letting you take your cats and dogs with you (or even import cows etc. for a farm), this is a good way to get rid of pesky Zombie Pigmen wandering near your portal on the Nether side -- shove them through, then later you can follow and deal with them at your leisure (without all their buddies ganging up on you).

Zombie Pigmen sometimes spawn near portals in the overworld, but only a few at a time.

Chunk loading and time
In single-player modes, or if distant from other players, moving between dimensions will cause the chunks around the area you left to be unloaded. This effectively stops time in the dimension you left, until you return. This affects all ongoing processes, including animal and plant growth, furnace smelting, brewing, and even primed TNT. This also means that when dying in the Nether (and respawning in the Overworld), your items will remain (lava and fire notwithstanding) until 5 minutes after you return to the Nether, or nearby regions thereof (the chunk update radius also applies in the Nether). Note that in multiplayer modes, a nearby player can keep the chunks loaded, so this may not apply.

Coordinate Conversion
Horizontal coordinates and distances in the Nether are proportional to the Overworld in a 1:8 ratio (1:3 in Xbox 360 version). That is, by moving 1 block horizontally in the Nether, players have moved the equivalent of 8 blocks on the Overworld. This does not apply on the Y-axis, despite the Nether having only 128 layers. Thus, for a given location (X, Y, Z) in the Overworld, the corresponding coordinates in the Nether are (floor(X) ÷ 8, Y, floor(Z) ÷ 8). (the Java floor method gives the largest integer less than or equal to the argument, so an X- or Z-coordinate of -29.5 becomes -30, not -29) Conversely, for a location (X, Y, Z) in the Nether, the matching Overworld coordinates are (floor(X) &times; 8, Y, floor(Z) &times; 8). However, in the Xbox version, the values are multiplied or divided by 3 instead of 8, because of the smaller world size.

Portal Search and Creation
Portals do not "remember" what portal they are linked to in the other world, but instead perform the following whenever a portal is used by a player:

First, the game converts the entry coordinates into destination coordinates as above: The entry X- and Z-coordinates are floored, then multiplied or divided by 8 (or 3) depending on direction of travel. The Y-coordinate is not changed.

Starting at these destination coordinates, the game looks for the closest active portal. It searches a bounding area of 128 horizontal blocks from the player, and the full map height (128 for the Nether, 256 for the Overworld). This gives a search area of 257 blocks by 257 blocks, at the full height of the dimension being traveled to.

An active portal for this purpose is defined as a portal block which does not have another portal block below it, thus only the 2 lowest portal blocks in the obsidian frame are considered. A single portal block spawned in and placed using server commands would be a valid location.

If a candidate portal is found, then the portal will teleport the player to the closest one as determined by the distance in the new coordinate system (including the Y coordinate, which can cause seemingly more distant portals to be selected). Note that this is Euclidian distance, not "taxicab" distance.

Prior to 1.3, only the coordinates 0-127 on the vertical Y-axis was searched, even for worlds with height limits above 127 after patch 1.2, so the search algorithm would not find any portals created above Y=128. This was corrected in 1.3, and travelling to the Overworld will cause it to search at the new 256 height limit for active portals. However, because of the closeness factor and the fact that portals cannot be built above 128 in the Nether without creative mode, portals will generally not link above 128 in the Overworld unless there are no portals below 128 relatively nearby. The distance computation between portals in range is a straight-line distance calculation, and the shortest path will be chosen, meaning the closest candidate portal will be chosen, even accounting for the Y difference.

If no portals exist in the search region, the game creates one, by looking for the closest suitable location to place a portal, within 16 blocks horizontally (but any distance vertically) of the player's destination coordinates. A valid location is one that has enough space to spawn a portal and is on solid ground. The game prefers to create the exit portal with the same facing orientation as the entry portal, but will check the other 3 directions as well. Regardless of orientation, the closest valid position in 3D distance is always picked.

Because the Nether is limited to 128 high, the search algorithm will not build any portals above Y=128, either in the Nether or in the Overworld. If you use creative mode to "build above" the nether, then a portal can be created above the 128 height in the Overworld, but the nether portal above 128 cannot be traveled to. The search algorithm does not search above Y=128 in the Nether for active portals as exit points.

Note that it is unlikely to spawn a portal above 128 in the Overworld unless a solid ground space exists above the 128 height near the destination coordinates already. Since the generator does not generate terrain above 128 even in the Overworld, a valid location will likely be found on the surface of the world for it to spawn the portal on. The exception to this is in an ocean biome, if you happen to pick a spot where no large caves exist under the ocean.

If there are no valid locations within the range, the game will force creation of a portal at the destination coordinates. The Y-coordinate will be clipped to between Y=70, and 10 blocks below the height of the dimension (118 or 246, respectively. Any blocks there (air, or otherwise) will be converted into a portal.  Such a portal has 4 extra obsidian blocks placed on both sides of the portal to prevent the player from falling.

This is described in more detail at http://www.minecraftforum.net/topic/345806-nether-portal-science/.

Implications



 * Likelihood of 2 overworld portals linking to the same Nether portal - Normal World portals that are within 1024 distance of each other on both X and Z axis are almost always going to link to the same Nether realm portal on initial construction because 1024 translates to a distance of 128 in the Nether Realm, and the game checks for existing Portals within 128 "radius" around the destination (the 257x257x128 box).

A less precise method would be to temporarily deactivate all portals within a 128 block "radius" from within The Nether. Through death or with the aid of a second player, entering a new portal from the Normal World will force the creation of a new portal within the Nether which the Normal World portal should prefer. This is not recommended as it limits how close Normal World portals can be placed due to the Zone of exclusions and can lead to unpredictable placement of the resulting portal.
 * Pairing portals - To setup pairs of Nether portals properly so that they reliably travel to each other, it is best to build both portals manually. Build at desired location X,Y,Z in the Normal World. Then travel to the Nether World. And then dig your way to X/8, Y, Z/8, and build a portal there.

If you wish to ensure that two portals link together, manually build portals as close as possible in all 3 coordinate axes. It doesn't have to be exact, or even all that close, if the player ensures that no other portals will be constructed in the exclusion zone created by the difference.
 * Zones of exclusion - The Nether portal spawning algorithm can only spawn portals that are within a 33x33 block column centered on the destination. This will often cause it to spawn a portal at a location significantly different than the corresponding location in the other world. The larger the distance between two linked portals, the larger the zone of exclusion. This zone is the area in each world where you cannot build another portal without breaking the link between the first two portals. One way to think of this zone is as spheres around each portal, each of a true radius equal to its distance to the other. For example, if the Normal world portal was at (0,50,0) and the Nether portal at (0,100,0), then each portal is 50 meters away from the other. In this (simple) case, if a Nether portal was built closer than 50 meters to (0,50,0), then the Normal World portal will now link to it.


 * 1-way long distance teleport - The portal choosing algorithm can be used for long distance travel by manual construction at carefully selected coordinates. If the player has a Portal in Normal world at (0,64,0) but makes a Nether Portal at (127,64,127) with its perfect Normal World pair at (1016, 64, 1016), then the portal at (0,64,0) will go to the Nether Portal correctly (1-way trip) because it is the only portal available within the 128 search distance along X and Z horizontal axes of the expected Nether portal position of (0,64,0). In about 15 seconds, the player can then travel 1436 meters in the Normal World. This specific form of fast travel by Portal is one-way, since the Nether portal will not find this Normal World portal. Given that a railway in the nether would need to span only 180 meters to go this distance, it is usually not worth making such portal links. However, it is theoretically possible to make a one-way ring of portals, with each Normal World to Nether jump going a long distance, but such a ring would easily be disrupted due to the huge exclusion zones created.


 * Non-exploit water ladder replacement. - The Nether Portal is an also entirely viable, two-way replacement for the water or conventional ladder.


 * 2-in-1 Nether Portals - It is possible to end up in a situation where a Nether Portal "randomly" places the player in 1 of 2 possible Normal World destination portals. This is simply because the Nether Portal has two effective coordinates as it is 2 blocks wide, say (X, Y, Z) on the left, and (X+1, Y, Z) on the right. If the player entered on the left side, (X, Y, Z) translates to (X*8, Y, Z*8) in the overworld and the game picks the portal closest to that. If the player entered on the right side, (X+1, Y, Z) translates to (X*8+8, Y, Z*8) and the game picks a portal closest to that point instead. This situation occurs when the Nether Portal's location is roughly equidistant between the 2 Normal World portals (within 8 blocks overworld distance difference). However, building 2 Nether Portals side by side is probably better for destination clarity than building a 2-in-1 portal. It is possible to span distances with pairs of portals in this way, though normally faster to simply walk through the Nether.


 * Spawning a portal in the air - It is possible for a destination portal (either in the Nether or in the Overworld) to spawn floating in the air. If your portal spawns in the air, it will generate a 1x2x1 obsidian platform in the front and back of the portal. This can only occur if there is no possible spawn location in the entire 33x33x128 column of search region to find a suitable spot to place a fresh new portal AND there are no existing portals within the 128 block "radius" to link to.


 * Obsidian generation - You can set an overworld portal to link to a Nether portal but not vice versa, by offsetting the two portals so that the Nether portal is farther than 16 away in X or Z in Nether coordinates. This type of connection can be used to generate unlimited obsidian for mining. Leaving the Nether portal will create a new portal in the overworld. Destroy this portal by mining it, return to the original overworld portal, and travel to and from the Nether again to create another new portal. Advantages include being able to mine above ground and with no danger from lava.


 * Structure finder - With some luck, making a portal underground, entering the Nether and make another portal out of the other's proximity may create a Portal in a Stronghold, a Cavern or even an Abandoned Mineshaft, all of which may contain a variety of rare minerals. (even Diamonds, but the starting portal must be built under layer 16.)

In 1.2, there was a confirmed bug in which the "portal search" performed during travel back from the nether will not find any overworld portal above a height of 128. Thus, it was not possible to travel back to the overworld and land at a portal which is above y = 128. The bug was corrected in 1.3, and Overworld portals above 128 can be traveled to. However, portals above 128 in the nether are not searched for and not considered active, and cannot be traveled to.

Bugs

 * Being in the middle of a Nether Portal will cause any Ice or Water not to display.
 * As of 1.3.1, Nether and End portals don't play the travel sound anymore.
 * Tossing lots of Splash potions into a Nether Portal, then entering will rarely cause a glitch where the player appears to be in the void, but does not take any damage from being in it. Reloading the world will cause Minecraft to crash.
 * On SMP you can sometimes stand in a Nether Portal without going through, though the world still spins and turns purple.
 * Entities such as items and mobs will not travel through nether portals on SMP servers.
 * Death in a Nether Portal results in the loss of all items that were being carried. They are not in the Nether or the Overworld to be found.

Trivia

 * Prior to the Alpha 1.2.0 release, Notch suggested it would be possible for Ghasts to rarely appear in the Overworld near a portal, but Jeb later stated that the feature has not been implemented . However, as of snapshot 12w22a, Zombie Pigmen occasionally spawn near Nether Portals.
 * If you warp from world to world, the direction you are facing is maintained (i.e. if you were facing east on the overworld and you warped to the Nether, you would still be facing east in the Nether). Also, if you enter a portal while flying in Creative, you will still be flying after you warp.
 * An automatically generated portal may be built at a 90 degree angle to the one you entered.
 * If you run out of Flint and Steel and disable all of your portals while in The Nether, it is still possible to reactivate a portal by having a Ghast's fireball hit the portal instead of you. Conversely, an active portal hit by a Ghast's fireball will be deactivated.
 * If you throw a snowball at an active portal's inside, it will break when it hits; treating it like a solid block.
 * When you look through an activated portal with water behind it, the water will not be visible which will make squid easier to see. Likewise, if you look through a portal with water in front of it, the portal blocks will be invisible.
 * Portal blocks are also invisible if they are looked at from behind another portal block, mimicking glass.
 * Portal blocks cannot be moved by Pistons.
 * If the game crashes while traveling through a portal, then your inventory will be deleted and you will be re-spawned at your spawn point.
 * If you die while within a portal block (e.g. by fire), the items from your death will spawn at your main portal. (This is the one that you warp back through in the Overworld. This only applies if you have more than one Overworld portal linking to the same Nether portal.)
 * The sound emitted by the portal decreases in volume and frequency with the distance from the portal, and appears to fade entirely with at least 15 blocks between the player and a portal block along any axis. The effect is radial.
 * In order to deactivate a portal without destroying it, one used to be able to pour a bucket of water or lava next to the portal, so that the liquid flowed into it. Water was only viable in the overworld, but lava would work to shut down portals in either world. While this function was removed in 1.0.0, placing a water source block inside the portal frame will still deactivate the portal.
 * Portals cannot be placed next to each other. This means you cannot create two portals adjacent to each other and have both lit up, which in turn means you cannot create a really long tunnel of portals for custom maps (e.g. pretend wormholes).
 * You cannot pause the game while entering a portal by pressing the pause button, however selecting another window (i.e. alt+tab) will pause the game.
 * In 1.8's Creative mode, the portal block can be broken like any other block. Upon shattering, it emits a glass sound.
 * When the pre-travel swirling animation is happening when in a portal, you break blocks at the same speed as if you were in water.
 * As of the 1.1 release, you still cannot activate Nether Portals in The End.


 * If you put four portals in a square formation, they will light up and work, but the formation causes some type of bug so you cannot take things out of your inventory. If you try to place objects in your inventory, they will become stacks of 64 and not just one, or however many you wanted to place.
 * If a dispenser fires a fire charge through an activated Nether portal, the fire charge will disappear on contact with the portal block.
 * Portals can be created without mining obsidian. with four iron ingots and flint. The ingots can be crafted into a bucket so that lava can be placed in select locations within falling water and create a portal Alternatively, the lava can be placed underwater, or a mold can be built and filled layer by layer.  (The last ingot is used for the flint and steel.)


 * There is a nether portal inside the minecraft sign on the Xbox 360 Tutorial world.
 * If a mob from the overworld enters a portal, they will float in the portal as if it were water. As of 1.4.2, mobs can enter the Nether through portals, although once you follow them into the Nether, they will warp back to the Overworld. This can be avoided by quickly leading/pushing them out of the portal.
 * You cannot travel through a nether portal while riding inside a boat.
 * If you throw an item into the Nether Portal, it'll disappear and be teleported into the Nether. Same thing applies if something is thrown from Nether to a portal going to Overworld.
 * As of 1.3, using a portal no longer spawns you in front of the portal on arrival, instead spawning the player inside the portal block and causing the "nausea travel" effect to begin, but no transportation happens due to cooldown.
 * If you fire an arrow into a Nether Portal and enter the Portal, the player will take damage upon transport from the arrow.
 * If you are in the overworld and you fire a flame arrow that hits TNT in the Nether the TNT will not begin its detonation until you walk through the portal. This does not happen in multiplayer.
 * You cannot open your inventory while in the portal.

Media

 * On 29 October 2010 PC Gamer released this video, showing a portal being constructed and used.
 * On 1 April 2011 Think Geek released this video to advertise one of their annual fake April Fools product the Minecraft USB Desktop Nether Portal.