Here's a snapshot from the blender! In this snapshot, world generation around the edges of an old world will now be adjusted for a smoother transition! This is the first iteration of this system which we expect to keep improving. Keep in mind that if you try it out now, the result will be saved to your world forever - which, as we know, is quite a long time. Better be safe than sorry. Make a backup before you try it!

Also - the space underneath existing chunks is now filled in, but only with deepslate. We will later be adding full cave generation in this place, so if you want that in your world, make a backup before you try it!

Due to some updates to terrain generation in this snapshot, seeds have shuffled once again so worlds will not look like they did in the previous snapshot. We expect world seeds to be stable after this snapshot.

Oh yeah, did we mention that it's a good idea to make a backup?

New Features in 21w43a

The edge between old and new world generation is now adjusted when you upgrade a world

Upgrading of old worlds

When generating close to old chunks hew terrain is adjusted to better match the existing terrain at the borders
In old chunks, if there is Bedrock at y=0, the column below will be filled with Deepslate
- Worlds upgraded in this snapshot will permanently have this deepslate, so if you want new caves under height 0 later, make sure to keep a backup before upgrading
The old Bedrock between y=0 and y=4 in old chunks gets replaced with Deepslate
A new Bedrock layer is placed at y=-64

Changes in 21w43a

The Priority Updates video setting has been renamed to Chunk Builder
Reduced the amount of flooded caves near river and ocean coastlines
Aquifer water levels change less often, so you'll more often have larger areas with the same water level. This means slightly fewer underground waterfalls and slightly easier underground boat travel
Lava aquifers are slightly less common
Restricted Big Dripleaf placement to Clay, Grass, Dirt, Farmland, Moss, Rooted Dirt, Podzol and Mycelium

Technical Changes in 21w43a

Some internal details of block and fluid ticking have been changed to improve save times. While it should behave exactly as it was, it might be a good time to test your redstone contraptions (on a copy of a world or with a backup, of course)
The Chunk format has been updated

World Data: Chunk Format

Removed chunk's Level and moved everything it contained up
Chunk's Level.Entities has moved to entities
Chunk's Level.TileEntities has moved to block_entities
Chunk's Level.TileTicks and Level.ToBeTicked have moved to block_ticks
Chunk's Level.LiquidTicks and Level.LiquidsToBeTicked have moved to fluid_ticks
Chunk's Level.Sections has moved to sections
Chunk's Level.Structures has moved to structures
Chunk's Level.Structures.Starts has moved to structures.starts
Chunk's Level.Sections[].BlockStates and Level.Sections[].Palette have moved to a container structure in sections[].block_states
Chunk's Level.Biomes are now paletted and live in a similar container structure in sections[].biomes
Added yPos the minimum section y position in the chunk
Added below_zero_retrogen containing data to support below zero generation
Added blending_data containing data to support blending new world generation with existing chunks

Fixed bugs in 21w43a

MC-131930 Deep warm ocean generates without coral and sea pickles
MC-156616 Badlands layers not generating properly
MC-217379 Pufferfish don't spawn in deep warm ocean
MC-236624 Deep Warm Oceans don't count towards the "Adventuring Time" advancement
MC-236970 Badlands strata is covered by orange terracotta at Y=160 and above
MC-238073 Decorators are independent of world seed
MC-238939 Stone can generate in the Nether
MC-239397 Lava pockets generate in icebergs

Minecraft Snapshot 21w43a