Underworld

Underworld is an Iris pack that preserves the shipping Overworld pack's terrain graph at the same seed and coordinates, then replaces its materials, biomes, caves, deposits, vegetation, objects, loot, ambience, and ecology with Nether counterparts.

Generation contract

• underworld is the active dimension. It keeps the Overworld height, continents, regions, biome selection, generators, caves, and coordinate inputs, while using environment: NETHER and coordinateScale: 1.0.
• upperDimension is intentionally empty, and the pack contains no roof dimension or upper-dimension resources.
• The active dimension uses full ambient block lighting and exact vanilla-Nether fog colors for each biome derivative. Its oceans, surface fluid, and enabled cave aquifers all resolve through the dimension fluid palette to lava; no lower pack palette places water or a waterlogged block.
• Lower terrain, cave, decorator, procedural, and object palettes use Nether terrain, vegetation, lighting, ores, and native Nether-structure materials instead of Overworld blocks.
• Cave materials and reachable cave objects contain no dirt or grass blocks. The former glowstone-dominant cave family now uses wavy Simplex contour stripes of obsidian and crying obsidian with glowstone limited to a sparse stripe accent; standalone glowstone surface and ceiling decorators were removed from those caves.
• Magnetics now selects between the same broad Magnetic Hollows, warped Flux Crystal Caverns, and narrower Polarity Grotto geometry as Overworld, using Basalt Delta, Soul Sand Valley, and Warped Forest materials and objects. Its nine floating-biome entries also share Overworld's variable vascular or crystalline tails, coherent edge-taper variation, and restrained wall warp instead of fixed-depth slab undersides.
• Every active surface biome exposes sparse Nether quartz through a coherent Simplex gate calibrated to roughly 2% of surface columns, or about 5.1 candidates per chunk before competition with the biome's other decorators. Accepted columns form irregular connected surface veins averaging roughly 5–8 blocks per cluster instead of independent one-block flecks. Underground quartz and Nether gold use Minecraft 26.2's 16/10 attempts, configured vein sizes, netherrack-only host rule, and band from 10 blocks above the bottom to 10 below the top. Ancient debris uses the vanilla large absolute-Y 8..24 triangular pass and small bottom-to-top pass, both with sparse candidate shapes, full air-exposure rejection, and Nether-base-stone hosts. Regional duplicate ore and gilded-blackstone deposits are removed.
• All lower biomes have unique custom biome IDs, exact derivative fog colors, Nether ambience, no precipitation, and one of the five vanilla Nether derivatives. Native derivative spawn tables remain the baseline ecology. A compact Iris ambient layer supplements them with derivative-themed Piglins, Zombified Piglins, Hoglins, Magma Cubes, Skeletons, Endermen, and lava-surface Striders; aerial Ghasts remain native-only so their vanilla placement rules are preserved. Global and per-chunk cooldowns keep the supplemental population bounded, and every spawn entry uses equal rarity for Bukkit/modded parity. Arbitrary fire and soul-fire surface decorators run at 30% of their former fire-placement rate without reducing the non-fire entries in mixed palettes.
• Fortress, Bastion Remnant, Nether Fossil, and Nether Ruined Portal generation remains native, preserving registered content, weights, biome eligibility, start logic, processors, entities, spawners, loot, structure-specific mobs, and locate behavior. The 1.1 structure-set overrides reduce Nether Complex spacing from 27 to 26 (about 8% more candidates) and Ruined Portal spacing from 40 to 38 (about 11% more); Nether Fossils remain at spacing 2 because legal integer spacing cannot represent a 10% increase. Non-Nether vanilla structure families are denied as a second guard against Overworld template blocks, and disabledExact denies only minecraft:ruined_portal while leaving minecraft:ruined_portal_nether native.
• The copied editable structure graph, external datapacks, Overworld mob roster, orphaned assets, and unreachable resources are intentionally omitted. The replacement entity and spawner graph is Nether-only.

The lower terrain layout is designed to be coordinate-for-coordinate compatible with the source Overworld graph when both worlds use the same seed. Palette changes alter blocks but not the lower generator links, noise fields, biome lists, height ranges, or coordinate transforms.

Install and validate

Current Iris builds install the Underworld beta release automatically at startup when underworld is absent. The release asset is a flat-root archive at https://github.com/IrisDimensions/underworld/releases/download/beta/underworld.zip; /iris download underworld uses the same asset. Manual installation remains supported by extracting or copying this entire tree as underworld under the Iris packs root:

• Bukkit/Paper/Folia: plugins/Iris/packs/underworld/
• Fabric/Forge/NeoForge: config/irisworldgen/packs/underworld/

On Bukkit-family servers, validate with:

/iris pack validate pack=underworld
/iris pack status pack=underworld

Create a disposable managed world with /iris create underworld_test type=underworld seed=1337. A managed iris:* world is not automatically the destination of vanilla Nether portals. To replace the selected save's actual Nether in place, back it up, run /iris replace minecraft:the_nether type=underworld, and restart once; Iris preserves the canonical Nether identity and seed while replacing its chunk store and generator. coordinateScale: 1.0 supplies the 1:1 ratio.

The beta ZIP contains only the active lower-dimension resources.

Source and credits

Underworld continuously tracks the sibling IrisDimensions Overworld repository for terrain geometry while retaining independent ores and Nether-specific materials and runtime content. The original pack credits Astrash, ArMiN231, Brian, Coco, Cyberpwn, Espen, K530, RaydenKonig, RepixelatedMC, and Strangeone101. See IrisDimensions-license.md for the source license.