Complex sentence: Or, like the submarine, it may prove not to be; in recent years--after their appearance in science fiction--carbon nanotubes have been produced which have the strength (though not yet the length) to produce a space elevator.
Split into simpler sentences: Or, like the submarine, it may prove not to be. Indeed over the last few years & after their appearance in science fiction, carbon nanotubes have been produced which have the strength (though not yet the length) to produce a space elevator.