World trees are mystical, giant trees whose branches support the heavens or reach through dimensions. Often their trunk is in the human realm while their roots reach into the underworld. World trees are present in many real-world religions. In fictional worlds they exist as common elements either as a portion of a belief system or as tangible structures that exist within a world. If they latter, they may be sacred trees and their growth, death, and renewal may mark important eras or be important for narrative reasons.