You were probably taught at some point that people in the time of Christopher Columbus all thought the world was flat. However, this is a myth that pervades history - most people knew the earth was a globe! (Source)

Goddamnit! I've heard that so often already.

And then I learned separately that even the Greeks already knew not only that Earth was round, but even its circumference at a pretty good accuracy.

These two 'facts' genuinely had me thinking we must have lost a ton of knowledge from the Greeks...