rat-tailed maggots in an old swimming pool

Rat-tailed maggots are the aquatic larvae of drone flies (Eristalis tenax), serving as essential decomposers that break down organic matter in stagnant, oxygen-poor water. Their, long tail acts as a breathing snorkel, while their presence indicates a healthy breakdown of materials, making them beneficial for breaking down compost.

But, more importantly, ewww.

https://www.reddit.com/r/whatisit/comments/1sg10l2/what_is_this_nightmare_organism_which/