Minecraft wiki entry for Nether_portals, associated text: The Nether portal is an also entirely viable, two-way replacement for the water or conventional ladder.[3] Note that if you want to travel a vertical distance of h from a point (X, Y, Z) in the Overworld to (X/8, Y+h, Z/8) in the Nether, there must be no other Overworld portal at any point (x, y, z) within a distance of 8*h from (X, Y, Z) (i.e. within the spheroid ((x-X)/8)² + ((y-Y)/1)² + ((z-Z)/8)² = h², note that Y is not divided by 8). That is, if you want to travel large vertical distances, there must be no horizontally close portal. (This holds for a portal from the Overworld to the Nether. The reverse direction (Nether to Overworld) was not discussed here.)