Green lights across las vegas, fast, tightly formationed and switching places, low flying and gone within 30 seconds.On holiday in Las vegas, on the night of 4/7/09, myself and two others were on the rooftop of our hotel having a drink, i had only been sitting facing toward the south side of the "strip" watching aircraft for five minutes when i seen it.I couldn't believe my eyes when these green lights travelling atleast twice or three times the speed of any aircraft in the area came into vision. They must have passed directly over the airport and were changing formations in ways no kind of fighter jets or anything could. Green lights, roughly 8 of them, circular, tightly packed and switching places but keeping in a tight formation, this was so distinct and clear, i wouldn't be surprised if aircraft or other people spotted this that night as the sky was clear and alot of planes and helicopters were around. Also as they were so low, probably on 2000ft, someone else must have seen what we did.I quickly said to the two people beside me to look and they too seen the lights, they moved so fast across the sky that they were gone over the hills to the west of vegas within 30 seconds, no noise came from this, which for the speed they were moving you would expect to hear something from it. Also seen a helicopter head of in that direction five mintues later, which seemed odd as no helicopters the whole week i was there seemed to head off in that direction.Watching the sky for strange things is not something i would often do, this was so real and i would have loved to have captured this on camera, it was truly unbelievable and i don't expect anyone to believe me but i know what i seen and it was to my eyes not of this world so to speak.