A couple weeks ago when my area of the 100 acre woods was receiving a lot of rain, manhole covers were floating away from all the water rushing through the storm system. Several cars were driving through the areas of high water and hitting those open manhole. It was amazing the amount of damage that was done to a cage. I couldn't imagine how much damage that would do to a bike and rider.
I work in the utility industry and we get a call at least once a week about a mh cover missing. We have a lot of 12 kv power lines that run through these manholes and I would never even stop over one now that I've seen what happens when a power line fails and the cover flies 10 to 15 feet in the air before it comes down. It could kill a person parked on top of it waiting at a light. Chances are it will never happen but it's always in the back of my mind.