Nether portal

A nether portal is a manufactured structure that acts as a gateway between the Overworld and the 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, by completing a ruined portal, 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, a lightning strike, or natural spread of fire to flammable material adjacent to the portal. Nether portals can be activated only in the Overworld or the Nether. They cannot be activated in the End and customized dimensions.

The fire must be the last placed block in the structure—a fire on an incomplete frame does not result in the portal activating upon the placement of the last obsidian block.

When a portal is used by a player, if no corresponding portal within range exists in the other dimension, one is created there; see and. There is an infinitesimal chance of failure for the corresponding portal to generate in the Nether, leaving the player trapped until death or until another player constructs a new Overworld 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, the wait time is one game tick ($1/20$ second) for the player to transfer between dimensions. 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. Unlike players, other entities travel through portals instantly, and once they reach the other side, there is a cool-down time for 300 game ticks (15 seconds), in which they cannot go through any nether portals. Therefore, an entity can only travel though nether portals again, once it is not touching any nether portal for 15 seconds.

Zombified piglins 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 in certain squares adjacent to the portals in the Overworld, not inside them. Zombified piglins spawned in this way have a full 15-second portal cooldown, meaning they can't go through the portal they are spawned in unless they leave the portal for a while. 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 fewer 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.

Since nether portals also spawn mobs which may wander through the portal, this means that whenever a portal is in a loaded and active Overworld chunk, the chunks around the linked portal in the Nether will occasionally be loaded, for 15 seconds at a time. While minor, this does mean that these chunks will slowly accumulate "active" time, which may be enough for mechanisms to operate, and items to eventually despawn.

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
When an entity starts colliding with a nether portal block, the game records the coordinates of the entity.

The game then converts those coordinates into destination coordinates as above: The entry X- and Z-coordinates are multiplied if the entity is in the Nether or divided by 8 if the entity is in the overworld, while the Y-coordinate is not changed.

Starting at these destination coordinates, the game looks for the closest portal point of interest (POI). That point of interest can be within 17×17 chunks in the Overworld and 3×3 chunks in the Nether 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 entity 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.

Portal creation
For players, 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 open their inventory or the chat window while standing in an active portal, and any other GUI (such as that of a chest or villager) is immediately closed when opened.
 * 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!".

Publicity

 * A LEGO Minecraft 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.