"*It's amazing how good governments are, given their track record in almost every other field, at hushing up things like alien encounters.
One reason may be that the aliens themselves are too embarrassed to talk about it.
It's not known why most of the space-going races of the universe want to undertake rummaging in Earthling underwear as a prelude to formal contact. But representatives of several hundred races have taken to hanging out, unsuspected by one another, in rural corners of the planet, and as a result of this keep on abducting other would-be abductees. Some have been in fact abducted while waiting to carry out an abduction on a couple of other aliens trying to abduct the aliens, who as a result of misunderstood instructions, trying to form cattle into circles and mutilate crops.
The planet Earth is now banned to all alien races until they can compare notes and find out how many, if any, real humans they have actually got. It is gloomily suspected that there is only one--who is big, hairy, and has very large feet. ..."
From: Hogfather by Terry Pratchett*
*Always read his footnotes. They are funny.