Many mathematical functions can produce fractals.
They often contain complex numbers and are iterated, ie., the function's output is re-entered as input and the function is recalculated a given number of times. Color coding of resultant values yields intricate, even chaotic, images with the basic fractal pattern repeated in smaller and smaller detail as one zooms in on a portion of the image. A jagged mountain's parts are similarly jagged all the way down to its jagged rocks and boulders. This same reiteration at successively smaller scales of magnitude applies throughout nature - from the vein structure in leaves to sea fans to neural networks. For this reason, fractal geometry can be used to produce surprisingly realistic computer graphic landscapes, of this world and beyond.