The universal question "Are we alone?" should be replaced with the question, "Have they made contact with us yet?" It would be extremely arrogant of humans to assume that we are the only intelligent life in the Universe, but then we are an arrogant breed.
Many ancient peoples have legends, which speak of astronauts and strange craft. Native American tribes from Canada to South America have stories that speak of visitors from other Worlds. Romany gypsies, who are thought to have originated in India, and Australian Aboriginal legends also have stories passed down by their ancestors telling of extra-terrestrials. One ancient African tribe's creation story, told for thousands of years, includes remarkably accurate astronomical knowledge of a star called Sirius B. Earth's astronomers did not discover this star, invisible to the naked eye, until the 1950s. Research has shown that histories passed down by word of mouth in societies, which have no written language, remain, in their essence, surprisingly and consistently true to the original events.
The Old Testament Book of Ezekiel in the bible has a description of a fiery chariot from which exited four humanoid creatures in suits of burnished metal. Other Old Testament stories speak of fiery chariots. Hebrew Scriptures and writings speak of visitations from creatures that were definitely not human. Ancient Mayan pottery and even cave paintings show astronauts.
There is too the legends of Atlantis, were the Atlantans aliens? The descriptions of their advanced culture, scientific knowledge and ability and their physical description certainly indicate that they may have been.
There are in many places around the World strange and ancient pictures on the Earth, which are only fully appreciated from the air. We have been told that they were made for the Gods but were they a welcome or a landing site path for ancient spacecraft.
Stories have emerged in modern times too of visits from creatures not of this World. Some stories are obviously not credible and invented by publicity seeking individuals. Some are products of an over-active imagination, or eating too much cheese for supper, the teller is not lying and is convinced that they truly experienced the events of which they speak. Some sightings of alien craft can also be explained away, for example the testing of a new and secret aircraft, for example many people who saw the testing of the Night Hawk Stealth Bomber aircraft were convinced they were seeing a flying saucer.
There are however, some reports of alien happenings and sightings of craft, which are difficult to dismiss. Credible and trustworthy people, who in their professions must observe events in a logical, dispassionate manner, and record those observations factually and scientifically, make these reports. They include civilian and military pilots, police officers, and scientists.
There are countless conspiracy theories that the World's governments, especially the United States government, know either that spacemen are visiting Earth or that they are actually in contact with aliens and keep those contacts secret for fear of panic amongst the populace or for military reasons. It is likely and logical that governments would keep such information secret for many reasons. Given human nature, maybe not all the conspiracy theories are true but it is possible, given some of the historical "evidence" of visitors from other Worlds, that alien visitation has been happening since the dawn of time
There are many reasons for disbelieving some instances of alien visitations. However, some stories and evidence are impossible to disprove. Is Sergeant Clifford Stone's story a figment of his imagination? Who can say whether it is or not? If Alien races were here, it is logical that the U.S. government would be, at the very least, investigating them if not making deals with them, if only to stop any other country from gaining any technological, commercial or military advantage. Whether there are really 57 separate species of extra-terrestrials all interested in our puny species remains a conundrum. It is clear that Seargeant Clifford only mentions 7 species, have the press got the story mixed up with one about "Heinz" 57 varieties?
There are those who will always be skeptical, but then we cannot see or touch love, we can only feel it. It is most unlikely that we are alone in the Universe. It is likely that Earth is being visited and has been visited in the past. Whilst you will not see E.T or little green men, it is possible that beings, who did not originate from this planet, walk unnoticed on the Earth.