The epic Ramayana, tells that King Janaka of Mithila found a baby girl while ploughing the field. He and his wife Queen Sunaina brought her up as their own daughter and named her Sita.
Actually, she is also considered to be a goddess. It is believed that Lord Rama is the incarnation of Lord Vishnu and she is the incarnation of Goddess Lakshmi. She came from Mother Earth or Bhudevi and towards the end of her life she went back into the lap of Bhudevi.