This mod changes the names of Underground Belts into "Underneathies". That's it. No more, no less.
Compatibility for Bob's Mods is also included.
The original Reddit thread that spawned this idea was posted by Dynamic_Gravity: https://www.reddit.com/r/factorio/comments/5yi071/need_your_help_in_settling_a_debate_with_gf
DisRuptive1 explicitly requested a mod that does this here: https://www.reddit.com/r/factorio/comments/5yxb09/mod_idea_change_the_name_of_underground_belts_to