Ashen Deeps

Born from a happy accident while messing around in modded 1.16 caves world, and "remastered" for 1.21.1, Ashen Deeps takes place in a subterranean Overworld entirely subsumed by the Undergrowth (more or less the Biomes o' Plenty biome of the same name, though with substantial retooling for the context).

The pack keeps it fairly simple, with Create providing most of the technology in the pack, and Silent Gear and Artifacts enabling a nice variety in gear. Some more detailed lists (dependency mods omitted):

No quest mods - you begin in a small starting structure with some logs you can salvage, but after that you're on your own. However, there are new structures you can find (villages in the ceiling and temples in the deeps), and many adjustments have been made to the experience to facilitate survival.

• In the early game, the wilds of the Undergrowth provide. The ubiquitous brambles can be smelted for charcoal or milled for string, and their foliage may rarely yield berries; additionally, bone blocks (found in nether fossils and the occasional bone cairn) can now be harvested by hand, and their bonemeal turns the ubiquitous cave vines into a food source. Not to mention the kelp!
• Also speaking of the early game, the Silent Gear blueprint bundle has been adjusted to provide more useful blueprints (prominently including the hammer, slingshot/cord, and shield).
• Once you've got a foothold, there are several processes that'll come in handy when expanding it. Magma blocks can be broken down into magma cream (slightly lossy) which can then be washed into slime or smelted to extract blaze powder, dirt can be smelted from soul soil, and sand can be milled from soul sand (which itself can be milled from soul soil).
• After getting properly established, you can begin to recreate materials of the past with the power of alchemy and arcana - many crops (like sugar cane, bamboo, flax and cotton) can be created alchemically (i.e. by mixing source plants with a potion base), while other objects like Ender Pearls and Overworld saplings require creation of Echo Shards, first through synthesis and then optionally through sculk farming from a synthetic sculk catalyst.

I put this together mainly to see if I could recreate the world in a newer generation of Minecraft and then scope creep happened. Hopefully if you got linked this or stumbled across it, it'll be neat for you too.