What are some facts agreed to by most humans, at one time in our history, that are now known to be wrong?

The idea here is to amaze everyone with your command of trivia. And to limber up the mind, break some boxes, shatter some glass ceilings, and start the process of challenging the basis for our beliefs.

O.K. I'll get things rolling.

Numerous scientists and engineers thought heavier-than-air flight was impossible. In 1895 Lord Kelvin stated, “heavier-than-air flying machines are impossible”. Of course the Wright brothers proved this wrong just eight years later.

In under a century we went from impossible to mach 3 flight, trips to the moon, and manned orbiting laboratories.

It's not that these scientists and engineers were not bright. All the facts they used to make thier claim were correct. It's just that certain critical facts were either not considered or not know at the time. Using the flat earth example, this was a very logical idea before there was an understanding of gravity. Once new information (gravity) is thrown into the mix then the idea of living on a ball instead of living on a flat plate begins to seem O.K. This is why keeping an open mind about what can and can't be done is absolutely critical. Once you close your mind, you fix the reality you live in.