So I have an Undertale AU named Scubatale that I've revamped a lil and I shall share here :3c
Sans and MC the lovers! Sans is based on a Mantis Shrimp, and MC is NOT Frisk or Chara she's her own character!!! Just gotta make that clear!
AU lore under the cut it will be LONG fyi
The 'Underground' is a bunch of underwater caverns with an abandoned research site around it all, all under the ocean, and some of the research labs and caverns are caved in or in a state of crumbling down because they've been abandoned so long.
The Ruins are an example as it has caved in and there's only a semi-stable hallway leading to a small stable cave with a small home where Mettaton lives! Waterfall is full of flowing rivers and high concentrations of saline. And Hotland is boiling so hot, with so many hydrothermal vents, that normal humans or monsters not accustomed to the heat have to go a long way around to get to the capital. Some monsters who need to get into Hotland can do a small enchantment that they have to consistently put onto themselves to get through, although the monsters living there are more than okay with inviting in monsters to their homes to cool down.
The CORE is always under repair as it's built over a giant hydrothermal vent that's started melting the tech inside. Which occasionally shuts the whole place down. So monsters have to stash lots of food and magic for when those times come around.
When a whole bunch of monsters have faith in two monsters within a specific cavern, that faith can oftentimes give them more power than they usually would have, and for boss monsters having the entire underground giving two of them faith they're essentially the closest thing to gods that they have. There are two people in charge of each area, Asgore and Toriel are King and Queen of the entire underground and are kind of divorced so their relationship is only business.
Alphys’s mind is twisted by the remaining presence of W.D. Gaster, essentially making her into a Gaster follower. She commits inhumane tests and experiments on the fallen monsters so when the families find out what Alphys was doing they get the information to Toriel and Asgore, which causes Alphys to be booted from her job as a Royal scientist and gets her shunned. However in her time as Royal Scientist, she managed to get Mettaton to distract Asgore and Toriel and she manipulated Asriel to give his magic to her sudden desire for immortality, and he agreed which caused him a severe and sudden magical deficiency. On top of looking for immortality, she also is trying to figure out a way to bring Gaster back into one piece.
With his magical deficiency, Asriel is stuck as a teen and it leaves him bedridden as well. However, he always tries to find ways to show Asgore and Toriel he's fine by sneaking out often. On the inside Asriel feels very guilty about being manipulated by Alphys so he's often trying to distract himself. With Asriel consistently sneaking out, Toriel commissions the new Royal Scientist, Sans, to make some sort of robot to locate and ensure Asriel's safety by bringing him home. So he makes the Flowey bots to locate and notify the closest Royal Guard of Asriel's location.
Undyne adopts Monsterkid and also mentors him in how to be a Mayor for Waterfall. She also hides Alphys in her house and only knows that Alphys was trying to help the fallen monsters, she has no clue about how inhumane Alphys was and refuses any ounce of knowledge that gets released. Although she's very aware that if anyone knows she's hiding the locally shunned Alphys, that her reputation will plummet, so she keeps it on the down low.
A portion of Waterfall is exposed to the ocean, thanks to some of the research lab collapsing into it. The barrier keeps anyone or anything from leaving. Still, fish often swim through the barrier and become trapped so eventually Undyne takes the opportunity, ironically enough, to have people capture any fish that go into or near the saline rivers and trade them with other locations or put them into the local food stash.
When Alphys and Mettaton were booted from their jobs and everyone there lost faith in them both, RG1 and RG2 gained mayorship over Hotland and they're absolute sweethearts. They were the ones that had a longer route around Hotland so no one had to worry about people or monsters being boiled alive.
Papyrus used to train with Undyne, but he found out she was hiding Alphys so he slowly stopped visiting her to train. He also became mildly uncomfortable after asking her about it and she sorta snapped at him. Papyrus and Sans don't have the heart to call the guards on Undyne, especially considering she was the Captain and she would likely talk her way out of trouble.
After Papyrus stopped going over for training, a machine was placed outside his door (by Alphys), when Papyrus opened the door he became blinded tripped on the rug, and fell backward smacking the back of his head on the hardwood. He had to lay there discombobulated as Sans stuck by his side and eventually, they figured out that Papyrus completely lost his vision. As a result of permanent sight loss, he has to buff up his natural internal magic detection and can detect magic (or lack thereof) in the water and he even picks up echolocation. He especially has vocal stims of clicks and whistles.
After all of that Papyrus rebuked his faith in Undyne as mayor but still kept contact with her until she attacked Sans which solidifies both of them in the decision to cut Undyne off for the time being for their safety. As mayors of Reefdin he and Sans, alongside some assistance from some robotic creations of Sans's, figure out the laws, keep track of food/magic stashes, taxes, etc. Sans does most of the paperwork, alongside being a Royal Scientist, Sans and Papyrus also go out to train together when they can they usually train with harpoons but Sans personally likes using a Machete though. With Sans being based on a Mantis Shrimp, he can cut through water like its butter so he usually holds back against Papyrus.
Some monsters of Reefdin are still particularly salty, pun intended, about Sans and Papyrus's old laws and what they both did in the past but most of them forgave the duo and still put faith in them, as they constantly evolve and learn how to be better mayors.
When Sans became a Royal Scientist he moved some things into the basement of his and Papyrus's house, as Alphys kept trying to break into the lab and he didn't want her continuing her experiments. He trusts his basement more than the lab because he doesn't want to have to teleport such a long distance to protect it all, he also beefed up the security of the house so he and Papyrus were safe.
Papyrus knows of the lab stuff in the basement and only requires his brother to have an occasional check-in to make sure his mind doesn't bust by the same thing that got to Alphys. Throughout the plot Sans gets more and more cranky and sleepy as Alphys gets Undyne into her breaking and entering schemes so when he goes to warn Undyne personally if they keep it up he's gonna call the guards, he and Undyne get into a bit of a tussle before they both retreat and lick their wounds. Also in the plot he aids in MC snooping around and gathers the physical evidence for them in his lab.
Sans also helps the fallen monsters recover from Alphys's awful experiments and gets them back to a sense of normalcy, always keeping them in contact with their families. He's very patient with them and gives them a whole lotta TLC before sending them back to their families, and even then he consistently goes around to check on them all when he can.
All the caverns and caves have some sort of hole in the ceiling for the human scientists and researchers to view the lives of monsters, as they saw monsters as nothing but science projects with no need for autonomy. Eventually, though, Humans outlawed magic as some human children were affected by the presence of magic in their parents giving them all some sort of aquatic traits. Which, of course, challenged the belief that magical entities didn't deserve autonomy and weren't sentient. Magical children are seen as freaks and often get made fun of and belittled.
