Minecraft wiki entry for Airlock, associated text: An airlock requires only two components, a block affected by gravity (such as sand) and torches (regular, soul fire, or Redstone). When placing an airlock, be sure to think about choosing narrow passageways and choke points, so that less time is spent on the project. For a building with multiple entrances, multiple airlocks may be needed to achieve optimal dryness. Airlocks are usually built directly across the hallway. Most sand-based airlocks are only 1 block thick, due to the fact that only 1 layer of sand is needed to stop the water.