A Deep Chasm
Type 1 Caves
Lava Caverns
with Cave Biome Size set to ExtraLarge
Underwater lake in 1.14.4
Cave opening into Floored Caverns
:0
Better Caves with Quark, Thaumcraft, and Botania
Better Caves Configuration
So many buttons!
Water caverns in 1.14.4
Underground River
Another set of steps bodes well; probably one of my favourite natural formations.
Taken by /u/ExtraBodybuilder
Big Boi Caverns
Caverns configured with a huge size, featuring Underground Biomes and Quark
Cave System
Floored Caverns 2
Floored Caverns
A reflecting pool, deep underground.
Taken by /u/ExtraBodybuilder
Better Caves with Quark
Underground Ocean
A little water and lava mixing.
Taken by /u/ExtraBodybuilder