I think aliens have been here but on extremely rare occurrences. I just think if we'd be seeing them as often as some people like to make out then we'd eventually get crystal clear images or videos of the spacecrafts in question. If there's crystal clear images or videos of UFO's i'd love to see them. I haven't seen one yet trawling through tons of them on youtube. The only clear images i've seen of supposed alien spacecraft were the Billy Meier pictures which turned out to be fakes.