There are a lot of supernatural things that cannot be proven. So how do you explain the pyramids in egypt? Who formed them the stones are way to big and heavy to be done by humans. What other explination could there be for that? Or the crop circles, how can that be explain? The crops fall and are shapped perfectly.
We cant know that intelligent life forms have visited this planet, but what about other life forms, like bacteria, insects or just primordial ooze! Maybe we have been visited by intelligent life forms, or maybe not, everyone thinks " oh, intelligent life forms must have visited us, what about space ships!" They might have caught a ride on a meteor as bacteria!
Many people believe that the aliens are always in intelligent life forms. However on Earth we also have less intelligent life forms like animals, insects but also bacteria and viruses. It wouldn't be weird that small organisms would have travelled to earth with meteors or other objects that have entered earth. I do not expect that aliens, who are expected intelligent, have entered earth. But I do think, and there is many evidence, that other life forms have entered earth. This might explain why animals get extinct.
The universe is billions of years old, and is so vast that it is hard to imagine that it is completely empty except for our little planet. We have only been around as a species for a few thousand years, which is an eyeblink in the life of the universe. Earth may have been visited long before humans dominated the planet. If that were the case, why would aliens take any interest in the planet at all?
With or without biblical text or science we have always had fascination with the stars and space beyond our own planet. Ancient civilizations used the sky to map travels, dedicate monuments (the Pyramids of Egypt, the Mayan temples, Stonehenge) to other beings that depict characteristics beyond the description of man. There has to be some logic in the ascension of Jesus to the heavens. Was he not then extraterrestrial because he did originate from "heaven"? What ever faith or belief we may claim, there is always going to be some question regarding what is out there beyond the moon and neighboring planets.
The greatest examples of 'Alien' craft visiting earth, were in the nineteen fiftees and sixtees, when the Governments of the World were conducting research into aircraft, on a dark night the SR-71 could be confused with an alien craft from its speed and shape. Many aircarft designs fit the discriptions of the so termed extra terrestrial craft.
Some people here seem to have been watching too many ridiculous episodes of "Ancient Aliens" and taking them seriously. There is no verifiable evidence of extra-terrestrial visitation.
Does some form of life exist somewhere else in the universe? Most probably. The sheer number of star systems and planets, including the 500 we've already discovered in the "Goldilocks" zone, and the Drake Equation make it unlikely we are alone in the universe. But are these lifeforms intelligent and capable of inter-solar system travel, and bothering to visit us? So unlikely as to be dismissible.
While I have no doubt that there is intelligent life out there, I don't think it's made itself known here even if it's aware we exist. The vastness of the universe and how perfect so many things have to be for life to form makes it unlikely anything is anywhere near us, just getting a one sun system is rare enough and once you get that you need millions of other things to work out. It's somewhe, it's probably not close.