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 (4×5 minimum, 23×23 maximum), after which the inside of the frame is ignited to activate it. The four corners of the frame are not required, but portals created by the game will always include them.

Behavior


Once a frame is constructed, the player can directly set it 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. A nether portal frame can be built in the End, but it cannot be lit/activated.

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, 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 the matching portal 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 then the player dies in the Nether and then re-enters the Nether. However, multiple portals can be exploited to farm obsidian.

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, inter-dimensional railways are limited to cargo. Note that mobs have a much longer "cool-down" time than the player, so they can't go back for approximately 30 seconds, by which time they will have wandered or been led away from the portal. Horses cannot be ridden through, but with the use of a lead can be brought through both ways, as can any other entity that a lead can be used on. Horses ridden into contact with a portal will move between worlds if the player dismounts.

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 Console Edition). 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 × 8), Y, floor(Z × 8)). However, in the Console Edition, the values are multiplied or divided by 3 instead of 8, because of the smaller world size.

Both the X and Z coordinates in this conversion are constrained to be between −29999872 and 29999872 (inclusive); this has effect when traveling to the Overworld from the Nether at X or Z beyond ±3749984.

Portal Search and Creation
Portals do not permanently "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, if the portal block in which the player is standing has been used recently, then it will re-use the destination that was chosen the last time; in this sense, portals do "remember" their linked pairs, but only for about 60 seconds (1200 game world ticks, or 600 redstone ticks). One side effect of this behavior is that the cached destination is not validated before being re-used, so if a player travels through a portal and immediately deactivates it on the other side, other players can still follow them through for the next 60 seconds and appear at the same destination, even though there is no longer an active portal there. After 60 seconds have passed without anyone using the same origin portal, the cached destination will expire.

If the player's origin portal has not been used recently, then a new destination will be computed. First, the game converts the entry coordinates into destination coordinates as above: The entry X- and Z-coordinates are multiplied or divided by 8 (or 3) depending on direction of travel, while 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 Euclidean distance, not taxicab distance. 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 3&times;4 buildable blocks with air 4 high above all 12 blocks. When enough space is available, the orientation of the portal is random. The closest valid position in 3D distance is always picked.

A valid location exactly 3 wide in the shorter dimension may sometimes not be found, as the check for a point fails if the first tried orientation wants that dimension to be 4 wide. This is likely a bug.

If the first check for valid locations fails entirely, the check is redone looking for a 1×4 expanse of buildable blocks with air 4 high above each.

If that fails too, a portal is forced at the target coordinates, but with Y constrained to be between 70 and 10 less than the world height (i.e. 118 for the Nether or 246 for the Overworld). When a portal is forced in this way, a 2×3 platform of obsidian with air 3 high above is created at the target location, overwriting whatever might be there. This provides air space underground or a small platform if high in the air.

Because the Nether is limited to 128 high, the search algorithm will neither find nor create portals above Y=128 in the Nether. Portals may be found or created above Y=128 in the Overworld if there are no closer portals or valid locations.

Once coordinates are chosen, a portal (always 4×5 and including the corners) including portal blocks is constructed at the target coordinates, replacing anything in the way.

If a portal is forced into water or lava, the liquid will immediately flow into the generated air blocks, leaving you with no airspace. However, a glitch can prevent this water from flowing into the portal: if liquid would flow both vertically and horizontally into the air pocket, it instead flows only vertically, so the blocks on the platform's outer corners never become water source blocks.

Implications



 * First portals are risky - Portals try to avoid spawning over lava, in midair, or inside rock, but they do so by spawning nearby. Thus, a new portal from the Overworld has a disproportionate chance of being next to an abyss, lava lake, or netherrack wall. There is also no check as to whether a lava source (created with the landscape) is destined to send lava flowing over the portal. Furthermore, a portal can spawn on a one block thick ledge or floor, or on a Soul Sand outcrop.


 * Farming method- you need 20-28 obsidian for this. Make a first portal in the Overworld. Go through it, and secure a base if needed. At least 16 blocks away, make and light another portal. This will take you back to the overworld, creating a portal some distance from your original portal (8 times the Nether distance). Take the trouble to secure the area where you come out, including fencing/walling off the area. Besides the risk of mobs wandering in, you don't want to come out and find yourself next to a creeper, or in the firing line of a skeleton. (Note that a creeper blast on either side will take out the portal!) Mine the new portal in the overworld for 14 blocks of obsidian. Go back to your main portal, and repeat as many times as you like. Note that if you destroy the Nether side of your main portal, your original portal in the Overworld will probably link to the new Nether Portal.


 * Portals can be built in networks - No more than 64 Nether blocks (256 Overworld blocks) apart, in the manner reminiscent of the gates in the Stargate series. You should build portals at (this is the maximum ideal distance, but they can be built as close to 16 Overworld blocks apart, if the co-ordinates are accurate) 64 Nether block intervals, even if you are not normally going to use these gates. This is so that if you use Nether portals for long distance travel, and your usual Overworld destination portal becomes inaccessible for some reason, (due to large gravel caveins, lava, water, or you have an automated activation system and forgot to turn it on) you will still have a reasonably close backup gate, which will get you back into your gate network.


 * Likelihood of 2 overworld portals linking to the same Nether portal - Overworld 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 257×257×128 box).


 * 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. 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.


 * Zones of exclusion - The Nether portal spawning algorithm can only spawn portals that are within a 33×33 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. 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.


 * 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. Note that if you want to travel a vertical distance of h from a point (X, Y, Z) in the Overworld to (8*X, Y+h, 8*Z) in the Nether, there must be no other Overworld portal within a distance of 8*h from (X, Y, Z) (i.e. (X/8)² + (Y/1)² + (Z/8)² = h², note that Y is not divided by 8). That is, if you want to travel large vertical distances, there must be no horizontally close portal. (This holds for a portal from the Overworld to the Nether. The reverse direction (Nether to Overworld) was not discussed here.)


 * 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 1×2×1 obsidian platform in the front and back of the portal. This can only occur if there is no possible spawn location in the entire 33×33×128 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.


 * 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 mine shaft, 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.

Gallery
 File:zombie pigman and nether portal.png|A zombie pigman wandered through a Nether portal and into the Overworld. File:Inside Portal Alpha 1.2.3 01.png|A view of inside a portal. File:Minecraft_Portal_23x23_Graduated.png#Summary|The biggest Nether Portal size (23×23 exterior, 21×21 opening) 

Trivia

 * An automatically generated portal may be built at a 90 degree angle to the one you entered.
 * 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.) They may not all land on the same end.
 * 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 can be placed together in a tunnel-like fashion (e.g. pretend wormhole), though it will only appear as if the third portal is lit as the first two in a row will mimic glass. If more than six portals are connected, the inner portals will be completely invisible while in the portal tunnel, however the particle effects can still be seen throughout. These connected portals also share the 4 second countdown until teleportation, so as long as you are within a connected portal you will be sent to another dimension.
 * Nether portals can also 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 enter to your inventory while standing in an active portal, however, you are allowed to scroll through your hotbar and place blocks (This can be very difficult due to the nether portal animation).
 * 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 Console Edition tutorial.
 * 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're on fire and travel through a portal the fire will be extinguished.
 * In the Console Edition, wolves will not go into the nether portal and go to the Nether; they will simply stay in the portal and not go anywhere.
 * 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 the Console Edition, then pause the game once the Nether is generated with the swirling effect still on-screen, the effect will stay on-screen.
 * A LEGO Nether Portal was included in the LEGO Minecraft Set: "The Nether".