TL;DR
Secret seeds in 1.4.5 are world modifiers. They do not just reroll terrain, they can reshape the surface, change visuals, or alter how the run plays. Start with Skyblock for a real challenge, then try Dual Dungeons, Planetoids, Mole People, and Waterpark for worlds that feel instantly different.
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How to use secret seeds
When you create a world, type a seed into the Seed field. You can enter either:

Capitalization and spacing are not important. If the seed is accepted, the game confirms it during world creation.
In 1.4.5, once a seed has been entered at least once, it becomes easier to reuse from a secret seed menu during world creation. Many of these modifiers can also be stacked, so start with one at a time if you want predictable results.
The special seed added in 1.4.5
Skyblock

Type: Special seed
What it does: You spawn on small floating islands with limited resources. Progress comes from bridging out, expanding carefully, and making every block count.
All secret seeds in 1.4.5
These are the 35 secret seeds introduced in 1.4.5, shown as phrase, code, and the practical effect.
| Seed phrase you can type |
Seed code |
What it changes |
| monochrome |
paintEverythingGray |
Grayscale world visuals |
| negative infinity |
paintEverythingNegative |
Negative color filter visuals |
| invisible plane |
coatEverythingEcho |
Echo-style visibility coating across the world |
| xray vision |
coatEverythingIlluminant |
Bright visibility coating that makes the world easier to read |
| mole people |
noSurface |
No normal surface layer, play is pushed underground |
| save the rainforest |
extraLivingTrees |
More Living Trees on the surface |
| the carebears movie |
extraFloatingIslands |
Extra floating islands and sky features |
| i am error |
errorWorld |
Error-style world variant |
| night of the living dead |
graveyardBloodmoonStart |
Starts with Graveyard and Blood Moon pressure |
| such great heights |
surfaceIsInSpace |
Surface generation shifts upward into space-like terrain |
| bring a towel |
rainsForAYear |
Near constant rainfall mood |
| abandoned manors |
biggerAbandonedHouses |
Larger and more frequent abandoned underground houses |
| how did i get here |
randomSpawn |
Randomized player spawn location |
| beam me up |
addTeleporters |
Teleporter-focused world structures added |
| too easy |
startInHardmode |
World begins in Hardmode |
| fishmox |
noInfection |
Removes normal evil infection worldgen |
| does that sparkle |
hallowOnTheSurface |
Adds surface Hallow without replacing evil or jungle |
| purify this |
worldIsInfected |
World generates fully infected |
| toadstool |
surfaceIsMushrooms |
Surface becomes largely mushroom-themed |
| sandy britches |
surfaceIsDesert |
Surface becomes largely desert with more pyramid-style generation |
| truck stop |
pooEverywhere |
World generation includes clusters of the infamous block variant |
| arachnophobia |
noSpiderCaves |
Spider caves removed from worldgen |
| more traps please |
actuallyNoTraps |
Traps largely removed from worldgen |
| rainbow road |
rainbowStuff |
Strong rainbow visual effects across the world |
| jagged rocks |
digExtraHoles |
Jagged surface terrain with more holes and fragmentation |
| planetoids |
roundLandmasses |
Rounded landmasses and unusual circular terrain shapes |
| waterpark |
extraLiquid |
Flooded world feel with greatly increased liquid generation |
| we don't even test for that |
portalGunInChests |
Portal Gun can appear in chests |
| winter is coming |
worldIsFrozen |
Frozen, winter-heavy world generation |
| pumpkin season |
halloweenGen |
Halloween-style world generation |
| hocus pocus |
endlessHalloween |
Permanent Halloween theme |
| jingle all the way |
endlessChristmas |
Permanent Christmas theme |
| what a horrible night to have a curse |
vampirism |
Vampirism-style ruleset, daylight becomes dangerous |
| royale with cheese |
teamBasedSpawns |
Team-based spawn setup |
| dual dungeons |
dualDungeons |
Two dungeons, one on each side |
| double daring dangers |
dualDungeons |
Alternate phrase for Dual Dungeons |

Quick picks, depending on what you want
If you want a world that looks different immediately, try monochrome, negative infinity, invisible plane, or rainbow road.
If you want exploration to feel unfamiliar, try dual dungeons, mole people, such great heights, or planetoids.
If you want a run that changes how you play, try too easy, random spawn, vampirism, or night of the living dead.
If you want chaos for building and screenshots, try waterpark, then stack it with a visual seed.