Picture, if you will, a pleasant evening in Detroit in early June. Crowds are picnicking on Belle Isle, watching a Tigers game at Comerica Park, and walking through Greektown. Suddenly, swarms of inch-long, wriggly insects emerge from their birthplace in the depths of the Detroit river and descend upon the city. They cover every surface, coating the streets and filling the air with a noxious, fishy odor. At their worst, they gum up machinery and coat the roads in an accident-inducing slick of bug carcasses.
As you might suspect given it's being described here, this is not a plot from a bad movie, but instead the miracle of the fishfly, whose breeding season spawns an annual plague covering Detroit and the eastern coast of the state (e.g. the thumb of Michigan's mitten). While the swarms vary in intensity from year to year (in part based on the oxygen levels in the Detroit River), at times they pile up so deep that homeowners and shopkeepers have to resort to shovels to clear them off of the sidewalks. The oddest part is that outside of the immediate vicinity, almost nobody in Michigan has the foggiest idea that this happens every spring.
Stories and Speculation
That ignorance could, of course, extend to fictional visitors to the city, which is where the fun comes in. The descent of a cloud of fishflies is a perfect tool for freaking out characters, complicating an otherwise mundane outdoor action sequence, or foreshadowing a major outbreak of the unnatural ("What the hell! Fishflies in December?"). For that matter, the fishflies can themselves serve as a physical manifestation of Otherness; perhaps the Snake Goddess of Belle Isle can be contacted by communing with the horde, or maybe a crashed UFO in the river uses the fish flies to search for signs of their jettisoned power core on an annual basis.