After the Valar entered the world, there was a misty light veiling the barren ground. The Valar took this light and concentrated it into two large lamps, Iluin and Ormal.
Aulë forged great towers on which to place the Lamps, one in the furthest north, Helcar, and another in the deepest south, Ringil. In the middle of Arda, where the light of the Lamps mingled, was the Great Lake and the island Almaren, where the Valar first dwelt.
The lamps were destroyed by an assault by Melkor, and the Valar fled Middle-earth for Valinor. At the site where Illuin fell, the inland Sea of Helcar was formed, of which Cuiviénen was a bay. According to the earlier writings of Tolkien, there was also the Sea of Ringil to the south, perhaps associated with the roots of Ormal.
Other Versions of the Legendarium
According to older writings not used in the published The Silmarillion, the Valar wanted peace with Melkor, and decided to ask his assistance: they wished to fix the lamps upon Arda's ground. Melkor, black with hate and envy of the rest of the Valar, agreed to help. He gave Aulë a substance which was both sturdy and strong: ice. Melkor bided his time and allowed the Valar to do as they wished until the fateful day when the Lamps' light and heat melted the ice and crashed upon Arda, flooding it with water and darkness.