Underilla is a Bukkit / Spigot based plugin for Minecraft Servers to 'merge' existing custom Minecraft word surfaces and vanilla undergrounds. It works by allowing the vanilla generation engine create chunks as normal, then intercepting the generator and forcing the surface of the original world, which works as a reference. In oder worlds, Underilla generates a brand new world with vanilla undergrounds, but cloning the surface of an already existing world.

It's original purpose is adding vanilla caves to custom WorldPainter worlds, but it would perfectly work for any pre-generated world.

Main features

• Three merging strategies:
  • None: No actual vanilla underground noise is generated. generate_noodle_caves setting can still be on, to generate noodle caves.
  • Absolute: A Y coordinate value divides the original world surface and vanilla underground.
  • Relative: This is the cool one. The reference world's surface will be dynamically carved into vanilla underground; which means there's no actual height-based division.
• Custom caves also supported. If using Relative merge strategy, every non-solid block and surroundings will be preserved, thus, if the reference world has itself an underground system, it'll be transfered over to the merged world.
• Heightmap fixed. Underilla re-calculates heightmaps when merging chunks, getting rid of floating and burried structures. Vanilla villagers and other structures are placed at the right height.
• Biome overwrite. Biomes from the reference world will be transferred and overwrite biomes from de vanilla seed being used. Cave biomes underground will be preserved.