I'm glad you asked! It's just a mod that adds the white ash particle from the Basalt Deltas biome to the underground to simulate dust! After all, the air wouldn't be 100% clear in a real mineshaft :p
Now to the nitty gritty:
It works by checking if the player has the sky above them while being below sea level and then scales the amount of particles based on depth.
It has a config system that you can access using Mod Menu, most things you can change in-game, but to change the particle you'll need to open the config file manually :)
Cave Dust requires the Fabric API but everything else Cave Dust needs is included. HOWEVER, I recommend Mod Menu to be able to use the config screen :p