That's rather the point, isn't it? At some point in time past, there was no way to detect it or understand it. "It" might have been observed, but with the knowledge base at the time, "it" was considered, wrongly perhaps, to be somehow supernatural. Words cannot always express things well, and so some with awesome math skills might use that to express or understand any given phenomenon, so "supernatural" was a term used to describe events, powers or phenomena man did not yet understand.
Tesla might be a good example. His understanding of electricity and power was unconventional and in the end "politically incorrect" for his times. But now we are beginning to partially understand that many of his far out claims about harnessing a natural and invisible power might have been spot on.
Pyramids are another example. The US has been a bit too busy exporting democracy to the rest of the world by violence, but the Russians spent many years studying the power of pyramids, and have built of number of them. What happens inside pyramids under certain conditions is very amazing. We cannot duplicate those things without the pyramid, and what happens inside cannot be filmed or understood, but it can be observed.
Is what happens inside the pyramid supernatural, or is it simply a natural power that modern man knows nothing about? If the Russians can duplicate experiments, I say it is a natural process that many humans know nothing about.
Semantics gentlemen, and definitions.