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 and are often incorporated into fictional religions as well. In fictional worlds they can exist as either a portion of a belief system or as tangible structures that can be physically interacted with. If the latter, they may be sacred trees and their growth, death, and renewal may mark important eras or be important for narrative reasons.