Minecraft wiki entry for Nether_portal, associated text: If that fails, too, a portal is forced at the target coordinates, but with Y constrained to be between 70 and 10 less than the world height (i.e. 118 for the Nether or 246 for the Overworld). When a portal is forced in this way, a 2×3 platform of obsidian with air 3 high above is created at the target location, overwriting whatever might be there. This provides air space underground or a small platform if high in the air. In Bedrock Edition, these obsidian blocks are flanked by 4 more blocks of netherrack on each side, resulting in 12 blocks of platform.