Minecraft wiki entry for Nether_portals, associated text: Portals try to avoid spawning over lava, in midair, or inside rock, but they do so by spawning nearby. Thus, a new portal from the Overworld has a disproportionate chance of being next to an abyss, lava lake, or netherrack wall. There is also no check as to whether a lava source (created with the landscape) is destined to send lava flowing over the portal. Furthermore, a portal can spawn on a one block thick ledge or floor, or on a Soul Sand outcrop.