Which would make them non-fantastic to the next generation, as such fantastic things as horseless carriages, man made flying machines, and missions to the moon are now.
[...horseless carriages, man made flying machines, and missions to the moon...]
...are all examples of things created by humans through a lot of hard work and ambition; they didn't fall on us from the sky by accident. But I get your point. It's a fact that there have been countless reports of stones falling from the sky dating back to antiquity, but scientists didn't accept that idea (and most openly ridiculed it) until perhaps a little over a century ago. Now, everyone accepts that stones fall from the sky as commonplace. Still, it took compelling evidence to bring about that widespread acceptance and there is no compelling evidence that supports the idea of aliens visiting Earth in spaceships.