Answer:
There are differing themes and plot devices: Space Travel, of course- and the related angle of attack/presentation of alien life forms. The so-called Ancient Astronauts and variant history theme - for example Atlantis, is related. I can recall a tantalizing end to a science fiction novel- where the lead character and his partner (using a special- viewplate-equipped submarine) located a dead and abandoned UFO of the space-station type- on the Ocean Floor in an Atlantis-like setting. That was how the story ended- they didn"t develop it further. also while we are on it- Time Travel fits in and in general ideas and technologies that are beyond (but are they?_ present day science. There are different types, of course- the artificial life angle is one of the oldest, dating to the Medieval Golem, and in more modern times, Frankenstein, Pinocchio.... and more modern robots, cyborgs, and androids.