Detected unsafe terrain read during worldgen in 26.2 (MC-307918) #13995

Stack trace

Both Vanilla and Paper have this issue when loading deep dark biomes.
See MC-307918.

This is not exactly a stacktrace or exception, is an error log that might get spammed without printing any stacktrace or interrupting the execution of any code.
Still, it prints a large wall of error logs that makes it hard to ignore, and will definitely send server owners not yet aware of this into a panic.

Log in Paper:
https://pastes.dev/x7sueyL2oN

Plugin and Datapack List

Datapacks:
None

Plugins:
Worlds

Actions to reproduce (if known)

To re-create on Paper:

1. Create a new server
2. Install a world manager, in this case Worlds
3. Generate a single-biome deep dark world using /world create test type single-biome minecraft:deep_dark
4. When teleported, fly around for a bit
5. Check the console

Seems to be related to the sculk propagating randomly, so it might take a couple of tries, although the single biome world should make it extremely common.
This is not a Worlds, or any world manager issue, it happens on a clean server with no plugins when using seeds like the following, but is way harder to trigger:

1. 8419721602848250140
2. -3343607211599936850

Paper version

This server is running Paper version 26.2-37-dev/26.2@f86d935 (2026-06-27T22:00:27Z) (Implementing API version 26.2.build.37-alpha)

Other

No response