Tutorials/Airlock

Airlocks are player built structures designed for the Indev Version of Minecraft. They can be used to effectively limit water from spreading into your base if your base is in danger of frequently flooding. These airlocks are one-way, meaning they will close in the event of flooding but will not reopen unless the player breaks them.

Airlocks can still be used in Beta, although reeds and doors can be used instead of sand and torches.

How They Work
Airlocks rely on the current interesting properties of torches, allowing the torches to support blocks of Sand above themselves until they are removed. In this case, when water comes into contact with the torch, the torch vanishes. This releases a large pile of sand, which in turn removes the support for the second torch. This cycle can continue for as many torches as you want and therefore works very well in most hallway sizes. This is also one of the very few reactions that can occur without a player's direct assistance.

Construction
An airlock requires only two components, Sand and torches

To place the airlock, simply choose a narrow (although it doesn't have to be) hallway or room to serve as a choke point, hopefully this room is the only entrance to the rest of your base, and directly between the water and whatever you wish to keep dry. If you have multiple entrances then you may need multiple airlocks to ensure optimal dryness.

The airlock is built directly across the hallway, and is usually one block in depth.

Now you will need an empty space directly above the hallway section equal to the number of blocks from the floor to the ceiling +1 per every block distance across the hallway (Every block more than 1).

So if you had a 3x3 hallway, you would require 9 blocks of sand. Then you would require one torch per block width of the hallway, for a total of three torches.

You would take the first torch and place it on either wall at the roof's height. This is the torch that will be triggered by the water and therefore MUST be at a level at which water will contact it.

Next, you would place three blocks (more or less depending on hallway height) of Sand or above the torch. The blocks of sand will be supported by the torch and will fall down to block the hallway the instant that water contacts the torch. (This is all you need if your hallway is only 1 block wide)

Now if your hallway is more than 1 block wide, you will need other segments to fall into place as well! Now this can be done in multiple ways, but the most effective is to have the first torch directly trigger the second column of sand to fall.

This is accomplished by directly placing a torch on one of the segments of sand that will fall. When the sand drops the torch no longer has anything it is attached to, and automatically release the successive column of sand. so, if it the hallway is three blocks wide, it would look like this:

=Other Uses= That is to say, they are an effective system for making Sand-Related things happen when water is introduced to an area. Making them one of the first completely environmentally reactive trap in Minecraft.

This concept could easily be adapted to other uses as well. Such as limiting the amount of water flow automatically by shutting off the water source when the water height reaches a torch, or changing the flow of water when the water reaches a certain area, thus "controlled area filling".

Example of Player triggered trap using the torch/sand concept The airlock works essentially the same, but faster and directly controlled by water flow, and not player interaction (Making it a completely standalone mechanism), Although this video proves that the method can be used to effectively trigger multiple walls and seal off multiple areas.

The torch system could also trigger sand falling into a room, where a hole in the ceiling then opens up and lava then pours in to cancel out the water. As long as it involves the movement of sand and water, it's quite an effective trigger system.

Uses in Creative Mode
By using flowers/mushrooms you may be able to build a less effective version of this airlock, since mushrooms/flowers are removed when in contact with a water/lava square. However the efficiency of simply using Trees to block water/lava makes sand based airlocks much less worthwhile.

When Survival Multiplayer is released, these airlocks may be useful to prevent water from coming through.

Trivia

 * Torches will work with both sand and gravel, but gravel must be placed (not dropped) on torches or it breaks (without producing any flint).
 * In Beta, Sugar Cane can be used as a type of airlock, though it would be a bit difficult growing reeds inside a building, especially underwater, though possible.
 * Natutally, you can use Gravel, and Redstone torches, too. However, they are harder to get.

Video Guide
http://www.youtube.com/watch?v=9hbWUJtGXSA&feature=related How to build an airlock]