Bukkit

With this plugin, the sky is no longer the limit! You can build upwards infinitely. Not only that, but this plugin lets you mine infinitely downward too!

How does it work? Watch this for a simple explanation: https://gfycat.com/SorrowfulRaggedIcelandicsheepdog

If you would rather read about it, then continue reading this. This plugin teleports you to a new world whenever you go below y = 0 or above y = 255. If you place a block at y = 0, it places the same block at the top of the world below it and vice versa. The plugin is very customizable: you can set a sky limit and an underground limit, you can disable elytra in the sky worlds, and more. The plugin is pretty straightforward and should be easy to understand once you get used to it.

How to install:

1) Download HiLo.jar.

2) Drop HiLo.jar into your plugins folder.

3) Let the plugin know which world is the default world by typing '/hilo setworld <world>'. If your default world is already named 'world', you don't have to do anything.

4) Enjoy (hopefully!)

This has not been tested with Spigot.

IMPORTANT INFO THAT MIGHT ANSWER YOUR QUESTIONS:
- For this plugin to work correctly, please define your server's main world by using the command '/hilo setworld <mainworld>'. If your default world is named 'world' you don't have to worry about it.

- To add caves, an admin must type '/hilo caves'. PLEASE NOTE: Adding caves will cause lag and make the underground worlds generate slower. Use at your own risk. I am still working to fix this.
- If you decide to enable caves, expect the underground levels to cause lag when you are transporting from one to another.
- THIS PLUGIN WILL NOT WORK WELL WITH A MULTIWORLD PLUGIN! When exiting the underground or sky worlds, it will always bring you back to the default world.
- If you have essentials and you set your home in a sky world, it will not bring you there by typing /home unless that sky world has already been loaded.

Permissions: hilo.admin

Commands:

'/hilo help' - Gives you a list of commands.

'/hilo setworld <world>' - Type this to let the plugin know which world is the default world. This is important for taking you back to the original world when exiting the underground or sky dimensions.

'/hilo caves' - Toggles cave generation. (CAUSES SLOWER WORLD GENERATION! Use at own risk.)"

'/hilo coords' - Tells you your coordinates with the absolute Y location.

'/hilo dungeons' - Toggles dungeon generation. (NOT ADDED YET)

'/hilo weapons' -  Toggles use of fun cave items found underground. (NOT ADDED YET)

'/hilo bedrock' - Toggles underground bedrock layer generation.

'/hilo groundlimit <number>' - Changes the maximum number of total underground levels.

'/hilo mincg <number>' - Changes the minimum level caves appear in.

'/hilo maxcg' - Changes the maximum level caves appear in.

'/hilo pick' - Toggles use of bedrock pickaxe. (NOT ADDED YET)

'/hilo elytra' - Toggles use of elytra in sky worlds.

'/hilo skylimit <number>' - Changes the maximum number of total sky levels.