World Seed

The World Seed feature enables the display of the random seed used to generate a Terraria world, and allows the user to input a custom seed manually when generating a new world.

Worlds generated with Terraria 1.3.4 and later will always have a seed regardless of settings; The experimental feature setting merely allows the user to see, copy, and input them. If a seed is not entered manually during world generation, a seed is automatically generated randomly. Worlds generated prior to 1.3.4 will never have a viewable seed: while they were generated with a random seed, that information was only used during world creation, and was not stored in the world file.

Worlds created after 1.4.0.1 will always be able to have their seeds edited upon world creation, regardless of the experimental feature setting.

Copying seeds from created worlds
Worlds with a seed display a small seed icon in the world list, allowing the user to see that world's seed and copy it to their computer's clipboard. This feature is experimental, and requires that  is set to "true" in the config.json settings. A copied seed can then be input when generating a new world, which results in a nearly identical generated world, provided it is generated with identical parameters (same size, same choice between Corruption and Crimson, and same choice of difficulty).

World seeds are formatted like this:.

Size:
 * 1, if World size is Small.
 * 2, if World size is Medium.
 * 3, if World size is Large.

Difficulty:
 * 1, if World Difficulty is Classic.
 * 2, if World Difficulty is Expert.
 * 3, if World Difficulty is Master.
 * 4, if World Difficulty is Journey.

World evil:
 * 1, if World Evil is Corruption.
 * 2, if World Evil is Crimson.

Identifier: The actual unique random seed.

This formatting can be used to automatically set all the initial settings of a world to be as desired.

05162020
05162020 is a secret joke seed that creates an extremely strange world upon generation. The number of the seed is the release date for 1.4.0.1: 05/16/2020. It is also unique in that worlds created with this seed are not identical - if two worlds are created with this seed entered, they will not generate the same.

While 05162020 is generating, the background on the Menu will become that of the Glowing Mushroom biome, the Moon will become a glowing face from "The Moon, Venus and Jupiter will be aligned to form a smiley face on May 16, 2020" (which is also the release date for 1.4.0.1), all clouds begin to resemble Redigit's head, the generation descriptions and tips become random numbers that change every tick (except when "placing traps"), and the outline of the progress bar rapidly flashes between Corrupt and Crimson. When the world has been created, its icon shows a tree infected with both world evils, as opposed to normal worlds, icons of which show a tree that is partially infected with one of them.

While 05162020 is active, all music is replaced with its counterpart from Terraria: Otherworld, both the Moon Lord Legs and Red Potion items can generate in Chests, the Party Girl replaces the Guide upon entering the world for the first time, both World evils and ore variants generate in the world, and the Dungeon generates underneath a "dead" living tree. The dungeon itself is a reference to the popular YouTuber, Chippy's Couch, and will generate with the Chippy's Couch furniture. The Jungle Temple is painted green, and the Underworld will be inverted (the middle third will be a lava lake and the two thirds at the edge will contain structures).

Not the bees!
While Not the bees! is generating, the generation descriptions all say "Generating Bees".

While Not the bees! is active, the entire world is Jungle, Hive Blocks replace Stone Blocks, Hornet Nests and Larva can appear everywhere, the starting NPC is the Merchant, Queen Bee spawns at the edge of the world, all Water is Honey and Hornets can spawn everywhere.

History
世界种子