Shifts the whole Overworld up 64 blocks: bedrock at y=0, sea level at y=127, build limit y=384. Terrain, caves, ores, biomes (incl. deep dark), ancient cities and trial chambers keep the same shape relative to each other, just 64 blocks higher.
Install: put shiftworld.zip in <world>/datapacks/ before the world is first created.
How it works: vanilla worldgen files are copied with every absolute Y value moved by +64 (dimension height, noise range, y_clamped_gradients, 3D noises re-offset via shifted_noise, surface rules, ore/feature height ranges, carvers, ancient city / trial chamber start heights). Nether/End content is left untouched. Regenerate with shift.py against misode/mcmeta 1.21.11-data.
Known limits (hardcoded in the game, can't be fixed by a data pack):
- Deep lava lakes/aquifers (vanilla lava below y=-54) become water.
- Ocean monuments still spawn at the hardcoded y=39, so they end up buried under the raised ocean floor.
- Some fine terrain/cave noise isn't Y-shiftable, so the world is equivalent but not block-for-block identical to a vanilla seed.