On street circuits like Monaco and Singapore, the manhole covers require welding shut prior to F1 races.

It's not for a fear of large rats AKA R.O.U.S. It's because the amount of suction created by a Formula 1 car when it travels over a road, is enough to rip a manhole cover off.

Image source: http://sports.xin.msn.com/photogallery.aspx?cp-documentid=4736957&page=4