I believe there are no such things as aliens or spacecraft that have crashed here however I believe it suits the government to perpetuate the myth while officially denying it for one simple purpose. They have developed (in Area 51) their own advanced craft based on anti-gravity technology of which there was a lot of talk about in the 50s but which suddenly mysteriously died. The genuine saucers that have been seen are in fact these secret flying crafts and nothing to do with aliens. Even the crash of 47 could have been an early experimental craft that crashed or, how's this for an idea, it might really have been a weather balloon!
Anyway, the conspiracy here is that the government deliberately feeds certain people, organizations this myth that it is alien technology to cover up the fact that it is actually themselves. One good example of a dupe that has been used is someone called "Lazarus" who I believe was deliberately led to see what they wanted him to see (eg: something made to look like an alien) and then let him "escape" to tell anyone who wanted to hear. If he had really seen something of value, don't you think he'd be dead by now?