Legacy Bedrock Preservation (1.18.2 Forge)

Have you ever updated an old Alpha, Beta, or other pre-1.18 world to Caves & Cliffs, only to discover that Minecraft removed the original bedrock floor from your old chunks and replaced it with deepslate?

Legacy Bedrock Preservation is made to stop exactly that.

What It Does

When a pre-1.18 world is upgraded to Minecraft 1.18, Minecraft uses its BelowZeroRetrogen system to generate the new terrain between Y=-1 and Y=-64 underneath existing chunks.

As part of that process, Minecraft normally removes the old bedrock between Y=0 and Y=4 and replaces it with deepslate, opening the old world directly into the new terrain below.

Legacy Bedrock Preservation prevents that bedrock replacement from happening.

Your original pre-1.18 bedrock stays exactly where it was, while Minecraft's normal below-zero retrogen is still allowed to generate underneath it.

This means your old world keeps its original bottom layer while still gaining the new 1.18 underground beneath it. If you eventually break through the preserved bedrock, instead of falling into the void, you'll find the new deepslate terrain and caves generated below your old world.

The mod does not replace Minecraft's retrogen system or generate the new terrain itself. It simply prevents Minecraft from replacing the original bedrock during conversion and otherwise leaves the normal 1.18.2 conversion process alone.

What About Player-Made Changes?

Existing player modifications in old chunks are preserved as well.

During testing, chunks were modified all the way down to Y=1 with bedrock removed, custom blocks placed, redstone components, glass, lighting, and containers containing items. After conversion, the original chunk remained intact while the new negative-Y terrain generated underneath it.

This means old basements, mines, builds, intentionally removed bedrock, and other modifications near the original bottom of the world can remain the way you left them.

IMPORTANT: BACK UP YOUR WORLD FIRST

Always make a separate backup of your original world before converting it.

Minecraft world upgrades are not something you should attempt without an untouched backup, regardless of whether you are using this mod.

Never test the conversion on your only copy of an important world.

How To Use It

1. Make a backup of your original pre-1.18 world.
2. Create a temporary Minecraft 1.18.2 Forge profile.
3. Install Legacy Bedrock Preservation in that profile.
4. Copy your pre-1.18 world into the profile's saves folder.
5. Launch Minecraft and open the copied world.
6. Allow Minecraft to perform its normal 1.18.2 world conversion.
7. Check your important areas and make sure everything converted correctly.
8. Save and close the world.

Your legacy bedrock is now part of the converted world itself.

Do I Have To Keep The Mod Installed?

No.

Legacy Bedrock Preservation is intended primarily as a conversion tool. It changes what Minecraft is allowed to do while the old chunks are being converted; it does not add custom blocks or other content that the resulting world depends on.

After a successful conversion, the world can be opened without Legacy Bedrock Preservation installed.

It has also been tested by moving a converted save back into the regular Minecraft Launcher and opening it in completely vanilla Minecraft 1.18.2.

Old Chunks and New Chunks

The mod only preserves bedrock that already exists in legacy chunks.

Chunks that were never generated in your original world are still free to generate normally when you explore them in Minecraft 1.18.2 or later.

Because Minecraft's world generation changed dramatically in 1.18, the border between an old chunk and a newly generated chunk may sometimes be visible. Terrain may blend surprisingly well in some places and look more obvious in others.

Legacy Bedrock Preservation deliberately prioritizes preserving your original chunks rather than changing them to hide these differences.

Updating Beyond 1.18.2

Legacy Bedrock Preservation itself is made for the pre-1.18 → 1.18.2 conversion step.

Once the world has been successfully converted and saved in 1.18.2, the mod is no longer required to preserve the original bedrock. From there, you can continue updating the converted world through newer Minecraft versions normally.

As always, make backups before upgrading an important world to another Minecraft version.

Tested With

Legacy Bedrock Preservation has been tested using Minecraft 1.17.1 worlds containing:

• Naturally generated legacy terrain and bedrock
• Player-modified terrain down to Y=1
• Partially and heavily removed bedrock layers
• Glass and other player-placed blocks
• Redstone components
• Containers with stored items
• Old chunks bordering newly generated 1.18.2 chunks
• Normal 1.18 terrain generation from Y=-1 to Y=-64
• Vanilla Minecraft 1.18.2 after the conversion was completed

The original legacy terrain remained intact while Minecraft generated the new below-zero world underneath it.