J. R. R. Tolkien came to feel that the flat earth cosmology he embodied in his legendarium would be unacceptable to a modern readership. In The Silmarillion, Earth was created flat and was changed to round as a cataclysmic event during the Second Age in order to prevent direct access by Men to Valinor, home of the immortals.[1] In the Round World Version, Earth is spherical from the beginning.
Tolkien abandoned the Round World Version before completing The Lord of the Rings, but later regretted this decision. He created a Round World Version, "The Drowning of Anadûnê", of the Akallabêth, the central story of the submerging of Númenor. He felt unable to proceed with it because the Flat World version was so deeply embedded in his mythology, with vitally important[2] symbols like the Two Trees of Valinor which could not fit in a Round World Version. He never resolved the dilemma, continuing to redraft his published works to make them compatible with a round world version for most of the rest of his life.
His son Christopher, editing The Silmarillion which he published after Tolkien's death, considered adjusting the text to comply with Tolkien's wish to return to the Round World Version. He decided against doing that, not least because the Akallabêth relies intrinsically on the Flat World cosmology.
Scholars have given at least three possible reasons why Tolkien should have felt the need for the drastic change to his mythology. Firstly, Tolkien believed that the Númenóreans would understand that a flat Earth was impossible, and so would not have passed on a story about it. Secondly, he felt that ordinary readers would find it impossible to suspend their disbelief in a flat earth with magical trees. Thirdly, his attitude seems to have shifted from feeling comfortable with mythology to wanting Middle-earth to be realistically historical.