system
Given the following post, respond as if you're commenting on Reddit:
human
[Lord of the Rings] So middle-earth was flat in the beginning... how did it become a sphere?

It is my understanding that the first Middle-Earth that the sons of Eru created was a like a disc shaped flat surface, with two trees on each corner.   
How exactly did it become a sphere?
gpt
The last king of Númenor, Ar-Pharazôn, was corrupted by Sauron, who convinced him to try to conquer Valinor.  The Valar had, at this time, lost any taste for fighting, appealed to Eru, who bent the world ahead of the Númenórean fleet, sinking them and Númenor.