I don't know about how that man-hole got there but I do know about another one.
The summer, 1964, I graduated from high school I worked for a road construction company. We were building a new 6-lane road into the east side of Madison, WI. One day I was assigned the job of clearing the gravel the grader left around obstacles while he was making the final grade. The operator was really good and he left me with little to do. So I had time on my hands and my mind wondered. I have always like a good practical joke and when I noticed an extra man-hole-riser complete with the cover sitting next to the road I thought what if.......
Well I rolled it out and set it in the middle of the road fully expecting the blade man to notice it on his next pass. He did not and I got called away to do other things. As far as I know it is still there. Just the riser and cover with no hole under it.