Mysterious pulsating slimy blob found in North Carolina sewer! I think that's about all we need to start with, right there.
(What is it actually? Well, those boring old party-pooping scientists are suggesting that it's a bio-film-covered
colony of
Tubifex tubifex worms. It moves because a single worm coils randomly, and stimulates the rest of the mass to do so, making it contract like a single muscle. Gee, sounds to me like a plausible motive ability for a horrible hivemind worm creature. ^_^ )