Minecraft wiki entry for Ruined_Portal, associated text: In Java Edition, ruined portals can be generated by the player by loading ruined_portal/portal_<1 to 10> or ruined_portal/giant_portal_<1 to 3> with a structure block. These ruined portals generate as they are stored, meaning they are not modified as detailed in the Generation section above. All portals generated this way create air pockets if generated in other blocks.