To me, it’s: That ancient people thought the Earth was flat.
We have records from around 430BC where Greek philosophers spoke of the Earth being a sphere. In 240BC the Greek astronomer Eratosthenes calculated the circumference of the Earth and was only about 2% out.
My curiosity was peaked so I googled for anyone else interested:
The Egyptians.
Islamic mathematician Abu Bekr ibn Muhammad ibn al-Husayn Al-Karaji.
described by Indian mathematicians as early as the sixth century