Ancient City Underground 🏛️

Let Ancient Cities Fill the Underground

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Introduction

Ancient City Underground is a Fabric mod that completely reworks Ancient City generation:

• All Biomes — Ancient Cities are no longer restricted to the Deep Dark. They generate in all Overworld biomes underground.
• High Density — Generation spacing of 10 chunks. With a 128-block city radius, every chunk is covered by city structures.
• Moderate Overlap — Cities overlap just enough for full coverage without excessive clustering.
• Safe Spawning — On first join, players spawn at a random safe spot inside the nearest city — a walkway, courtyard, or other open area.
• Bedrock Roof — A bedrock layer at Y=60 seals the underground world. No blocks or mobs generate above it.
• Fully Compatible — Does not affect End Portals, Strongholds, or any other vanilla structures.

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Features

Feature	Description
🗺️ All Biomes	Generate under plains, deserts, forests, oceans, mountains — every Overworld biome
📐 Optimized Density	spacing=10, separation=2, ~160 block average spacing, 128-block coverage radius
🎲 Random Spawn	Each new world spawns you at a different spot — courtyard, walkway, or city gate
🛡️ Safety Check	Automatically verifies ground is solid, body and head are in open air
🧱 Bedrock Roof	Bedrock layer at Y=60 with no blocks or mobs above — a sealed underground world
⚡ Lightweight	Pure datapack overrides + Mixin, no vanilla structure definitions modified

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Versions

Two versions are available:

Version	Features	JAR
Full	Ancient cities + spawn + bedrock roof Y=60 + no mobs	ancient-city-underground-full-1.0.0.jar
Classic	Ancient cities + spawn only	ancient-city-underground-classic-1.0.0.jar

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Requirements

• Minecraft: 1.21.11
• Loader: Fabric Loader ≥ 0.19.3
• Dependency: Fabric API ≥ 0.141.4+1.21.11
• Java: ≥ 21

This is a SERVER-SIDE mod. It works on dedicated servers, and clients do not need to install it. For single-player, it works as normal.

Installation

1. Install Fabric Loader
2. Download Fabric API
3. Download the mod JAR
4. Place the JAR into .minecraft/mods/
5. Launch the game and create a new world

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Technical Details

How it works

Mechanism	Method
All-biome generation	Datapack override: has_structure/ancient_city.json → #minecraft:is_overworld
Generation density	Datapack override: structure_set/ancient_city.json → spacing=10, separation=2
Player spawn	Mixin into PlayerList#placeNewPlayer — finds nearest city, picks random safe position
Bedrock roof	Mixin into LevelChunk constructor — clears blocks above Y=60, places bedrock at Y=60
Mob restriction	Mixin into NaturalSpawner — cancels spawns above Y=60

Structure Set Parameters

{
    "placement": {
        "type": "minecraft:random_spread",
        "spacing": 10,
        "separation": 2,
        "salt": 191119
    }
}

Parameter	Value	Description
spacing	10	Grid size in chunks. One city per 10×10 chunk cell
separation	2	Minimum distance from cell boundary
Min center distance	64 blocks	Closest two cities can be
Max center distance	256 blocks	Farthest two cities can be (edges just touch)
Coverage radius	128 blocks	Vanilla max_distance_from_center, unchanged

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Screenshots

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Compatibility

• ✅ Ancient Cities in all Overworld biomes
• ✅ No vanilla structure definitions modified
• ✅ End Portals unaffected
• ✅ Strongholds unaffected
• ✅ Other structures unaffected
• ℹ️ Only affects newly generated chunks (requires new world)

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License

This project is available under the CC0 1.0 Universal license. Feel free to use, modify, and share.

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Made with ❤️ by xiaomaiya