Nether portal

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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 (4x5 minimum, 23x23 maximum), 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. This means if you want, you can make the corners out of gold or diamond blocks and even dirt. Of course, you can use obsidian if you prefer.

Behavior


Once the frame is constructed, then the player can directly set the frame on fire by means of flint and steel or a fire charge, or they can indirectly ignite the frame by using lava. This creates portal blocks inside the frame, resembling a vortex. While a Nether portal frame can be built in the End, it cannot be lit/activated.

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. 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. Portal blocks emit light of level 11, rather dimmer than a torch.

When a player in the Overworld or the Nether 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. However, 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.

Much like water or lava, portal blocks cannot be broken (or even targeted) by tools. However, portal blocks can be destroyed by explosions (even weak ones) or water. If any portal block is destroyed, the whole portal is deactivated. TNT, a Ghast's fireball explosion, an exploding Bed, a Wither's explosive Head or an exploding Creeper/Charged Creeper can all disable a portal. (As usual, the explosions will not affect the Obsidian frame except for blue Wither Skulls.) Water from a bucket, a broken ice block, or whatever other source can also disable a portal, though of course not easily on the nether side as the water will evaporate. 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. Combining these deactivation/reactivation facts, it's possible to construct devices using Dispensers behind a portal frame to turn portals on/off using a button or other switch. Another way to deactivate a portal is to mine or destroy any non-optional block of the frame; in this case, the portal cannot be re-lit until the block is replaced.

Entities can travel through portals, including mobs, thrown items, and even empty boats and minecarts. Storage Minecarts and Powered Minecarts can pass through, but minecarts, boats, or horses with passengers (mob or player) cannot. Thus, interdimensional railways are limited to cargo. 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.

Zombie Pigmen occasionally spawn near portals in the overworld, but only one at a time. They spawn twice as often on Normal difficulty as on Easy, and three times as often on Hard difficulty compared to Easy. No other mobs can be spawned by Nether Portals in this way, in any dimension.

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. Note that a portal above Y=128 in the Overworld will generally not be found unless there are no lower portals nearby. The distance computation between portals in range is a straight-line distance calculation, and the shortest path will be chosen, counting 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. A 3&times;2&times;3 volume will be replaced with air, obsidian, and portal blocks.  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 In the "Portal Science" and "How Portals Work" links below.

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 making 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.)

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.

Issues
Sometimes on servers you might teleport into lava or teleport into nether without portal to head back if there is no receiving portal.

This generally happens because the receiving portal is almost completely surrounded by solid blocks. But can also cause people to appear within solid blocks too, on some versions of Minecraft.

Trivia

 * If you warp from world to world, the direction you are facing is supposed to be 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). However, there is a long-standing bug that causes your facing to be flipped 180 degrees while in creative mode.
 * 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.
 * 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.)
 * If you die and attempt to respawn in the portal, you may create multiple Nether Portals in the Overworld
 * 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.
 * 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).
 * However, nether portals can share obsidian blocks on their left and right sides, allowing for example the creation of two adjacent portals using only 17 obsidian blocks (Although creating two adjacent portals serves no real purpose).
 * 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 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.
 * Portals can be created without access to a diamond pickaxe, by casting the obsidian in place.
 * There is a nether portal inside the Minecraft sign on the Xbox 360 Tutorial world.
 * If a mob riding in a minecart from the overworld tries to enter a portal, they will float in the portal as if it were water. However, if the player tries to ride a minecart through the portal, they will simply pass through the frame without shifting dimensions.
 * Projectiles sent into a Nether Portal will be transported, but will not complete their trajectory until or unless the chunk on the other side is loaded. In single-player, this won't happen until the player comes through themselves. For example,  if you fire an arrow into a Nether Portal and then enter the Portal, you can be hit by your own arrow.
 * If you are in the overworld and you fire a flame arrow that will hit TNT in the Nether the TNT will not begin its detonation until you walk through the portal.
 * If you stand in an unactivated Nether Portal and light it, you will be instantly teleported (barring load times).
 * If you're on fire and travel through a portal the fire will be extinguished.
 * In the Xbox 360 edition of minecraft, Wolves will not go into the nether portal and go to the Nether, they will simply stay in the portal and not go anywhere.
 * As of Version 1.5, a bug causes primed TNT to not travel through nether portals. (https://mojang.atlassian.net/browse/MC-8983)
 * There is a splash referencing the Nether Portal. It says "Slow acting portals!". This is also a reference to the amount of time you have to wait in the portal before it teleports you to the Nether.
 * If you enter a portal in Xbox 360 Edition, then pause the game once the Nether is generated with the swirling effect still on-screen, the effect will stay on-screen.
 * It can be both annoying and deadly if you have a Nether Portal inside your house because Zombie Pigmen can spawn, and they can be hard to get rid of if you don't have the right equipment, unless you are on Peaceful.