There is an urban legend in NYC that a giant alligator lives in the sewer system under the streets. I don't know how true this is but every time I walk by a manhole cover, I think of the alligator.
But seriously, manhole covers do not pose any danger to pedastrians except during heat waves or underground fires. They cause the underground pressure to build up so quickly that the manhole covers pop up into the air and land on someone/something.
So do you have an urban legend to share?