My favourite underground overhaul mod. Adds some really interesting and atmospheric underground biomes that make mining and exploring under the earth much more enjoyable.
Combined with mods that add interesting mobs, like Goblins to your game, it makes for a great survival experience.
Will echo FaceDeer (the creator's) recommended mods: definitely add mods that add things like ropes, gliders, handholds or other ways of getting down some very, very, very deep and spooky caverns. Otherwise you might fall a lot, or have a hard time getting back out again.