Minecraft wiki entry for Nether_portals, associated text: It is possible for a destination portal (either in the Nether or in the Overworld) to spawn floating in the air, or buried in netherrack or stone. If your portal spawns in the air, it will generate a 1×2×1 obsidian platform in the front and back of the portal. If it spawns in rock, a 3-high airspace will be cleared out for one block on each side of the portal. This can only occur if there there are no existing portals within range to link to, and there is no suitable spot to place a new portal within 16 blocks horizontally (at any height) of the target coordinates. This usually means that floating portals spawn over the ocean (in the Overworld) or lava ocean (in the Nether), and buried portals are much more likely in the Nether.