According to Buddhist cosmology the universe is impermanent and beings transmigrate through several existential "planes" in which this human world is only one "realm" or "path". These are traditionally envisioned as a vertical continuum with the heavens existing above the human realm, and the realms of the animals, hungry ghosts and hell beings existing beneath it. According to Jan Chozen Bays in her book, "Jizo: Guardian of Children, Travelers, and Other Voyagers", the realm of the "asura" is a later refinement of the heavenly realm and was inserted between the human realm and the heavens. One important Buddhist heaven is the "Trāyastriṃśa", which resembles Olympus of Greek mythology.