Since I write and read urban fantasy, I spend a lot of time thinking about the different ways people would react when learning about an expanded world.
There are already a lot of believers out there: 42% of Americans and 52% of UK residents believe in ghosts, 57% of Americans believe in psychic phenomena and about 50% of Americans believe in some kind of conspiracy theory, including theories about secret societies and hidden populations.
I think we all have our own area where we keep an "open mind" even if we openly scoff at what we consider to be others' "out there" beliefs. Strictly from the numbers, at least half of the US population believes there is more out there than science currently explains. I believe that means the population as a whole would be quite ready to accept something new and beyond their current experience, provided there was some kind of proof. 70% of people polled believed in the Discovery Channel's fake Megalodon documentary, despite having been taught the massive prehistoric shark was long extinct.
However, I think each person would have a different reaction to the proof. Some would automatically deny it, looking for evidence of special effects or fakery. Some would do their own investigation to learn the truth, keeping an open mind. Some would accept the basics but refuse to go any further, lest they be branded as crazy by their peers. And some would accept it wholeheartedly.
It's not easy to believe in something which is publicly mocked and derided, even if a person has personal evidence to support it. It's easier to keep their personal experience quiet but that leaves the person feeling isolated and alone. It's a one-way door, once a person steps through, they can't ever go back to the comfort of mutual ignorance, yet they cannot drag other people after them.
Some people are like Mulder in X-files, they want to believe and will ignore evidence to the contrary. Some people struggle to fit what they've seen into the worldview they've always had, telling themselves they've hallucinated, misunderstood or been tricked. And some find themselves convinced despite all their efforts to avoid it.
But we can't avoid new truths forever. Eventually the evidence mounts beyond the ability to deny, taking us further than we could have ever imagined.