Give an example of a theory scientists believed for a long time that was eventually proved false

There are so many to choose from, I'll give you two. One is a big one. The other so/so. People used to believe the earth was flat and you would fall off, until Christopher Columbus sailed across. And another interesting one is the vulcan planet theory, which is that there was an extra planet between Mercury and T sun. however it turned out to be false. So there, need anymore just tell me.