Nether portal

A  nether portal is a manufactured structure which acts as a gateway between the Overworld and Nether dimensions.

Creation
A nether portal is built as a frame of obsidian (4×5 minimum, 23×23 maximum). The four corners of the frame are not required, but portals created by the game will always include them. The obsidian can be placed in any manner, e.g. by placing mined obsidian or by casting it in place using lava and water. Adjacent nether portals can share obsidian blocks.

Once a frame is constructed, it is activated by fire placed inside the frame. This creates portal blocks inside the frame, resembling a vortex. The fire can be placed in any manner, including use of flint and steel or a fire charge, impact of a ghast or blaze fireball, or natural spread of fire to flammable material adjacent to the portal. Nether portals cannot be lit/activated in the End.

Portals are created in the other dimension when the player uses one, see below for more information on this process.

Behavior
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. Note that wolves cannot travel through portals in the Console Edition. 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. If the chunk on the other side of the portal is not loaded, entities passing through (including projectiles) will effectively be held in suspended animation until the chunk is loaded.

Zombie pigmen have a chance to spawn on the bottom frame of the portal in the Overworld if any nether portal block above receives a block tick. 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.

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

 * 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.
 * 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).
 * There is a splash referencing the Nether Portal. It says "Slow acting portals!".
 * 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.

Publicity

 * A LEGO Nether Portal was included in the LEGO Minecraft Set: "The Nether".
 * 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.