So, a plane didn't hit the Pentagon on 9/11?

The one thing that i dont understand about consipracy theorists, esp. those pertaining to 9/11, is how they believe that the events of the attacks were really government plots.

In micheal moore’s film he tries to show that the pentagon was really hit by a missil instead of a plane for example. I don’t understand how they believe that something like this is utterly possible to not be uncovered by the media. “oh yea , its the government, they can cover anything up!”. Well, no. Something that complex and indepth, would involve so many people, resources and branches of government that it would come out. There is no way that it would not come out to the media. Someone would come forward, or something would be accidentaly uncovered. Common sense tells someone that covering up a plot this involved would be impossible.