UnderGround by PenguinMaster420, MCreator
A mod that adds the Underground update nobody asked for!
UnderMod
MOD BY UNDER
UnderUtilities by Loyto
Tab serveur + player skin + chat
Underarmor
Armor that goes under your players skin.
UndercastMod by skipperguy12, Genius_Fraser, GoldBattle, palechip, xIGBClutchIx, flv92, molenzwiebel
A custom made mod that tracks gameplay stats and has custom guis made for the oc.tc servers
Undercraft by ThePsionic
Minecraft meets Undertale
Underfell
This is still a demo but this is what I like to call the brother update
Underground Biomes by Exterminator Jeff, Grom PE
Adds underground biomes composed of layers of new rock types
Underground Biomes Ore Integration by Harley O'Connor
Extra ore integration for underground biomes.
Underground City Engineering by beanflame, GoldMain, Mr_zhao
Underground Ocean by Reoseah
Small mod that adds large caverns filled with water to world generation.