You mean they didn't think the earth was flat in ancient times? That's an urban legend?
Who is "they" The top thinkers in Alexandra knew the Earth was a sphereoid. In fact Eratosthenes the curator of the Library at Alexandria computed the circumference to within 7 percent of its current measured value.
Anyone he lived on the sea coast know the Earth was not flat because of the way ships appear and disappear at the horizon.
Even the rulers of Spain know the Earth was spherical when Columbus came around trying to raise funds. They initially rejected his proposals because they thought the distances would be too great. It was Washington Irving who created the myth that even the learned folk believed the Earth was flat.