If intelligent aliens visit the Earth, it would be one of the most profound events in human history. Surveys show that nearly half of Americans believe that aliens have visited the Earth, either in the ancient past or recently. That percentage has been increasing. Belief in alien visitation is greater than belief that Bigfoot is a real creature, but less than belief that places can be haunted by spirits. Scientists dismiss these beliefs as not representing real physical phenomena. They don’t deny the existence of intelligent aliens. But they set a high bar for proof that we’ve been visited by creatures from another star system. As Carl Sagan said, “Extraordinary claims require extraordinary evidence.”