Hypothetically, if one person is contacted/meets extraterrestrial life, they are said to be crazy or lying. Nobody believes them. Even though many people in the past claim to have met aliens, the majority of the world discounts their experiences.
So, the question is, how could the world be convinced of alien life in one event?
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Actual existing Aliens would be a good starting point, most people who claim to have been abducted also happen to be lunatics half the time.