An atheist could write about supernature topics, from the point of scientific research into the subject.
For example: An atheist could write about spells, and all the scientific research proving there is no such thing and how confirmation bias works. Or how what one religion calls "spells", another calls "prayer".
Did you actually mean "supernatural" or merely "unexplained"?
Anyway, I'd start with sky mirages. Usually these have some pseudo-scientific explanation (atmospheric lenses and so on), but sometimes they appear to reflect not merely another place, but also another time. That is sufficiently difficult to explain that no-one has tried it yet.
You don't have to be an atheist.
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