Whenever the first alien breached the atmosphere, preferably it was prior to the human race turning the planet into a never ending heaving schoolyard jamboree, whenever they come on down now they're wondering when it becomes necessary to quarantine us off permanently, they'll send our rockets bouncing off a big cosmic glass wall or just drop a DWACKLECACKAKOOKOO.334.-EFFHEFFALUMPADUMPA on us all which I believe means KABOOM and spreads DOOM.