ModsJava
Unstable Underworlds plugin
Java mod listed for Minecraft 1.21. Downloads from the MC Java Mods app on Android.
This plugin allowed 1.21.11 paper servers to have the famous unstable SMP underworld region. Free Minecraft Java mod for 1.21, 1.21.1, 1.21.2.
⬇ Download on Modrinth
Quick facts
- Edition: Minecraft Java
- File type: .jar
- Minecraft versions listed: 1.21, 1.21.1, 1.21.2
- How to install: Install the matching mod loader (Forge, Fabric or NeoForge) for your Minecraft version. → Download the .jar. → Put it in the .minecraft/mods folder and launch that loader profile.
- Where to get it: Opens on Modrinth — not every file is mirrored on our own servers.
Install steps are the general flow for this file type — How to install Minecraft Java mods & modpacks walks through it step by step.
File checked by MCModsHub
- Ships as UnstableUnderworld-1.18.2.jar — drop this file into the mods folder
- Download size: 288 KB
- Download link checked 25 Oct 2026 — working
These come from our own check of the pack file, not from the source page.