Large numbers of people believe in demonstrably false phenomena, from UFOs and ESP to Bigfoot and the Loch Ness monster. Even though these fictions have been repeatedly debunked and discredited, they persist in the human imagination and influence our beliefs and our society. Spinning tales of fantastical creatures may seem like a harmless pastime, but when pseudoscientists make “revolutionary” claims about the world and its history, evidence-based science, public policy, and human progress suffer.