How did you do that? Manhole covers are made of cast iron so they can withstand a great amount of pressure. This article on removing manholes also says you can use a crowbar to remove a manhole cover, however, it is best to buy a specialized manhole cover hook from a reputable hardware supplies store. Because you can seriously injure yourself if you attempt to remove the lid on your own, they advise you to call the citys engineers office and have them do it.
Imo, if manhole covers could easily be removed by hand, people would be throwing things in them left and right; therefore, to prevent this from happening, they are purposely designed so they are too heavy and awkward to lift without the use of proper equipment. In the City of Winnipeg, youre not allowed to remove a tree branch if its on City property. Because manholes are on City property, I dont think a person is allowed to remove them on their own. jmo
http://www.howtodothings.com/home-garden/how-to-remove-a-manhole-cover