A Gizmodo writer stumbled upon this video from 2012 that may have been "buried" at the time for a reason: that being, it's a horror show. YouTube user gurutek was in the process of breaking down his saltwater tank when he discovered the gigantic worm (which he calls a Eunice/bristle worm, though commenters at both Gizmodo and YouTube are arguing over the terminology) after two years of keeping the tank. If that story isn't bad enough, when he tried to grab the worm with tongs, a section of its tail snapped off and was still wriggling around.
Such worms can, when small, hide in rocks or coral and go unnoticed, eventually growing to be quite large. They are nocturnal, so therefore are easy to miss. Says gurutek,
"I was breaking down the tank (as I was moving it), hence it looking shoddy. That unice was in there 2 years before I noticed, and only noticed because I had whole coral colonies missing after a single evening. I first saw it after I spent a few nights sat up (after lights went out) for about 3 hours per night looking for the critter who was eating my corals. Even when I knew it was there, I only ever saw it 3 times within the space of a year. It hides in the rocks, and only comes out at night, impossible to catch without taking everything out the tank."
I used to keep a freshwater aquarium and had a plecostomus grow to a size that scared me. One night I had a nightmare about going on vacation and coming back to find horrifying, prehistoric looking creatures inhabiting my tank. I used to laugh at myself for having that dream. Now it's not so funny.