According to Greek mythology, Gaia was a goddess, who arose from Chaos. Chaos was the darkness that reigned in the beginning of existence. The omnipotent Gaia gave birth first to the immense light blue Sky, the Mountains and the Sea. The Sky after it covered the world, united with Gaia, creating all the other elements. Their marriage is the source of life of the world.
Sky and Ground is the fundamental pair, the immortal pair that gives birth initially to the Ocean, the river of rivers and myriads deities that express big forces of Nature and phenomena. Nereids, Titans, Giants, Mermaids, Medouses, Pigasos, Cycoples, Oceanides, thousands sons and daughters that give a face to known and natural phenomena and bodies, as the seas and the lands, the mountains and the rivers, the sun and the moon, the day and the night.
The bigger part of this first cosmogony is found in the work "Theogony" of Isiodos.