In many a scripture in the bible they refer to the earth having a foundation.
Some of the writings where the earth is described as shaking and rumbling, one can easly say they meant the earth settled down. Probably from a local earth quake or other act of nature. "The earth settled on it's foundation" This is within reason.
But for the most part, the people of Mesopotamia had no idea yet that the earth was round. How could they?