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That's pretty freaky stuff. It would be weird to think of not just one mutated creature, if the stated cryptozoologists are right and there is a race of them that has managed to stay in myth for 30 years now, they could be a dangerous guerrilla threat if real.

If they actually did find there way into the sewers like some of the cryptozoologists theorized in one of those links, then cars going over the roadways and such might agitate them if they do in fact exist and cause them to exhibit defensive behaviors. I know that if I go over a sewer grate or manhole cover, it makes a fair bit of noise, in a sewer system, that could echo pretty good. Then there is just the average sound that goes on in settled areas that can be heard from a grate, but that would have minimal residual effect in the sewers.