To start let's remember what the Ley Lines are and what my theory on Dark Mud is. First the Ley Lines are the life blood of Teyvat. They house all of the collected memories from everybody who ever lived. Dark Mud is a toxic material related to the Abyss. It powers them up just like Abyssal Energy. In my theory Dark Mud is Genshin's version of fossil fuels.
The connection here isn't quite scientifically accurate. The fossil fuels of our world come from plant matter from millions of years ago. After dying and being buried the pressures inside the Earth break it down into sludge that becomes petroleum or it solidifies with other rocks to become coal. But this makes the link between Ley Lines and Dark Mud work out because the Ley Lines function as "dead matter" for Teyvat.
On top of that it adds to the story. After all we know that the Ley Lines also act like a filter.
Unlike how our planet works Teyvat takes some cues from the Gaia Hypothesis where the planet itself is conscious and works with all the creatures living on it. In this case it filters out the negative influences of anything that returns to it so that that stuff doesn't get reborn into the next generation. Then in terms of Genshin's lore under normal circumstances this negative sludge will eventually be purified on its own. But if there's a large enough concentration of it - say if Rhinedottir's being naughty and sends a bunch of corrosion wolves or an "iron meteorite" at a place - then there's too much sludge for the Ley Lines to deal with. It will still filter it out but all of it will collect into masses of Abyssal Energy that won't be purified on their own.
Instead you'll need Celestia's Light Element to combat the Abyss Void Element. Of course now MiHoYo's written themselves into a corner. You see I still haven't done the Yougou Cleaning quest to completion. So if I were to take this shiny new Lumenspar Adjuvant down to the bottom of Yougou I should expect it to do wonders against that tumor right MiHoYo? Right? 
Tips and Tricks from a Super Casual Genshin f2p
