The answer is already there in your question: demons.
According to the Bible, several thousand years ago, the entire planet was under water. Most modern people refuse to believe this because it suggests a supernatural explanation.
Demons are a silly superstition that only scientifically backwards people could possibly believe. We're "enlightened" these days and don't accept such obvious nonsense.
Seriously, just do a little digging into how willing most people these days are to accept "supernatural" occurrences, and you'll have all the answer you need.
For bonus points, you probably have a religious fringe that does believe in what really happened (and gets laughed at by the rest of the "scientific" community). Boy, are they going to be smug when it turns out they were right all along. Assuming, of course, they don't all come down with an acute, demon-induced case of discorporation.
To expand on this a bit... (Please try to keep an open mind and try to avoid falling into politics; there is a relevant reason why I'm trying to explain this, which I will summarize at the end.)
What a lot of people don't understand about "science" is that your interpretation of the evidence is extremely dependent on your fundamental axioms (dogma). Creationist dogma is "the Bible is probably accurate". From this starting point, they are able to build a cohesive explanation of science, including plausible (to them) explanations for "evolution", the fossil record, and radiometric dating. This framework is capable of making useful predictions and even addresses some evidence that is problematic for uniformitarianists ("dark" matter, cosmic temporal horizon, certain inconsistencies in radiometric dating).
Uniformitarianist dogma, on the other hand, can be summed up as "God can't exist". This is not an evidence-based conjecture, it is an axiomatic assertion that uniformitarianists will not allow to be challenged. Because of this philosophy, they reject certain hypotheses that a creationist would take as self-evident, and are able to build their own, completely different, but still plausible (to them) view of the universe. This framework is also capable of making useful predictions and even addresses some evidence that is problematic for creationists (radiometric dating, apparent age of the universe).
Obviously, these can't both be correct, and yet good luck trying to convince either side that they're wrong. For that matter, try going after a real nut (flat earth, moon landing hoax, etc.) and see just how effectively they respond to evidence that is contrary to their belief.
The point, and the relevance, is that humans are really good at interpreting evidence to fit their preexisting dogma. For the purposes of this question, if humans somehow arrive at the dogma that demons can't exist, it doesn't matter what evidence there is to the contrary; they will find a way to explain it that doesn't violate their prior dogma. Their ability to do so is isn't conjecture, it is thoroughly demonstrated in real life. (It will help if there aren't a lot of written records immediately following "the event", but even that is no guarantee. Just look at how many cultures have a flood myth, and ask someone at random if they believe there was a global flood at some point in Earth's history. Now replace "flood" with "demon invasion".)
The secondary point and corollary is that you can take a mountain of inspiration for this from real life. Depending on where you want to take the story, it could easily become a social commentary for oppressed minorities, casting the "demon deniers" as the "scientific" majority and the "demon believers" as a marginalized, ostracized, and even persecuted minority. There is more than enough real world experience to draw on for inspiration.