According to mythology, Leto, pursued by the incensed and jealous goddess Hera, wandered from place to place seeking some corner of the earth in which to give birth to her children, the fruit of her union with Zeus, father of the Gods. But all the Islands and cities refused to receive her, afraid of the vengeance of Zeus' deceived consort, whom only a bare rock in the middle of the tempestuous sea dared to defy.- Source.
Prior to this event, the island (Delos) had floated aimlessly in the Aegean, but became anchored in its position by Poseidon who made four granite columns rise out of the sea to anchor it firmly in its present place for the divine birth. This is why the island came to be called Delos, which means 'visible', before which it was a floating, nomadic rock called Ortygia or Adelos (the invisible).
It is said that it was on Delos that Leto finally bore the twins, Apollo and Artemis under a palm. In return for the sanctuary of the island, she promised that the god she was about to give birth to would turn this dry and barren island into a place of great pilgrimage and bring prosperity to its land