Originally a flat world, the continents were surrounded by a mighty ocean, Ekkaia or the Encircling Sea, and separated by Belegaer, called the Great Sea. In the First Age, the area north and west of Middle-earth was occupied by the country of Beleriand, but this was destroyed during the War of Wrath.
In the Second Age, Númenor was raised in the Great Sea for the Edain. This island existed through most of the Second Age, but was destroyed as a result of the pride of the Númenorean people in defying the Ban of the Valar and sailing to Aman in the west.
After the destruction of Númenor, Arda was made round. Aman was taken out of the world, and could only be reached by the Elves, following the straight road that was granted to them. As Aman was taken away from Arda, new lands and continents were created.