Nether portal

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

Creation
A nether portal is built as a vertical, rectangular frame of obsidian (4×5 minimum, 23×23 maximum). The four corners of the frame are not required, but portals created by the game always include them, resulting in 4 free/extra obsidian. 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 portals can share obsidian blocks. A nether portal cannot be built horizontally like an End Portal.

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, the impact of a ghast or blaze fireball, or natural spread of fire to flammable material adjacent to the portal. Nether portals cannot be activated in the End and customized dimensions.

When a portal is used, if no corresponding portal within range exists in the other dimension, one is created there: see. There is an exception to the above rule. There is an infinitesimal chance that a portal created in the Overworld does not generate a portal in the Nether, which would leave the player trapped until death or construction of a new portal.

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 immediately transfers between dimensions. (Not on high velocity because the server has to have the player end up in the portal in a tick, not between ticks.) If there is already an active portal within range (about 128 blocks) in the other dimension, the player appears in that portal. Otherwise, a portal is 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 (unless there is another active portal within range). The usual cause for this is when the player's Nether-side portal is deactivated by a ghast, and then the player dies in the Nether, spawns and then re-enters the Nether through the Overworld-side portal. However, multiple portals can be exploited to farm obsidian.

Most entities can travel through portals, including mobs (except the wither and ender dragon), thrown items, and transportation without passengers (neither mobs nor player), including boats, minecarts and horses. Storage minecarts and powered minecarts can pass through. Thus, inter-dimensional railways are limited to cargo. Note that mobs have a much longer "cool-down" time than the player, so they cannot go back for 300 game ticks (15 seconds), and during that interval, they can wander or be led away from the portal.

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 as on Easy. No other mobs can be spawned by nether portals in this way, in any dimension.

Chunk loading
Whenever an entity is teleported through a nether portal, the chunk at the linked portal gets load ticket with load level of 30, meaning that it is fully loaded and can process entities. This load level also spreads to adjacent chunks but they get lower for each chunk. This results in 8 more fully loaded "entity ticking" chunks with gradually less loaded chunks further out.

These chunks remain loaded for 15 seconds but this timer gets refreshed each time the entity passes through the portal. This can be used to permanently load chunks, creating a "chunk loader". Permanently-loaded chunks created using chunk loaders create a considerable amount of lag.

Coordinate conversion
Horizontal coordinates and distances in the Nether are proportional to the Overworld in a 1:8 ratio. 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 to the Y-axis. 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)), and conversely, for a location (X, Y, Z) in the Nether, the matching Overworld coordinates are (X × 8, Y, Z × 8).

The Java floor method used in these conversions rounds down to the largest integer less than or equal to the argument (toward smaller positive values and toward larger negative values), so a coordinate of 29.9 rounds to 29, and one of −29.9 to −30.

Both the X and Z coordinates in this conversion are constrained to be between −29,999,872 and 29,999,872 (inclusive); this affects travel to the Overworld from the Nether at X or Z beyond ±3,749,984.

Portal search and creation
Portals do not permanently "remember" what portal they are linked to in the other dimension, 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 re-uses the destination that was chosen the last time; in this sense, portals do "remember" their linked pairs, but only for about 15 seconds (300 game world ticks, or 150 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 expires.

If the player's origin portal has not been used recently, then a new destination is 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 the 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 17×17 chunks centered on the chunk containing the destination and the full map height.

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

If a candidate portal is found, then the portal teleports 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 the range is a straight-line distance calculation, and the shortest path is 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 the 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. $$, these obsidian blocks are flanked by 4 more blocks of netherrack on each side, resulting in 12 blocks of platform.

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 immediately flows into the generated air blocks, leaving the player with no airspace. However, a glitch can prevent this water from flowing into the portal: if the 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.

Video
Note: These videos do not mention that Nether Portals on the Nether Ceiling link up with overworld portals.

Trivia

 * Portals can be placed together in a tunnel-like fashion, though it appears as if the third portal is lit as the first two in a row mimic glass. If more than six portals are connected, the inner portals are 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 the player is within a connected portal, they are sent to another dimension.
 * The player cannot enter their inventory while standing in an active portal or type in the chat; however, they are allowed to scroll through their hotbar and place blocks (this can be difficult to do with accuracy due to the nether portal animation being similar to the nausea effect).
 * If 2 portals intersect, and the player lights a fire in the intersection (i.e., in both portals at once) only the portal that lies within the x-axis is activated. Lighting any other block activates whichever portal it is in.
 * There is a splash referencing the Nether Portal. It says "Slow acting portals!".
 * Similarly to how end portals can generate already activated, it is possible (though extremely improbable) that a nether portal can generate naturally if water and lava flow create a portal frame and then a fire starts in the portal due to lightning or a nearby flammable block catching on fire from the lava.

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 products: the Minecraft USB Desktop Nether Portal.