neplex/stratum

Stratum is a Minecraft mod that stacks several separate dimensions on top of each other inside one continuous world, instead of keeping them apart as separate worlds you travel to through portals. Normally, moving from the Overworld to the Nether means stepping through a portal and waiting for a new area to load. With Stratum, you can dig straight down through the bedrock at the bottom of the Overworld and come out directly into the ceiling of the Nether below it, with no portal and no loading pause in between. The mod works by giving each dimension its own fixed range of height in the combined world, so each dimension's rules, such as which biomes, structures, and terrain features can appear, only apply inside its own assigned vertical slice. This keeps the different dimensions from generating on top of each other or mixing their world generation rules together. All of this is configured through datapacks, which are the standard way Minecraft mods and worlds define custom content, rather than through hardcoded settings. That means a server or modpack owner can set custom heights for each layer and decide the stacking order without editing the mod's code directly. Stratum was built specifically to support mods like Create: Aeronautics and was made for a modpack called Create: Above the Clouds, where flying and vertical movement between layered areas is part of the intended gameplay. The README does not mention a license.