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40d:Channel
A channel is a hole dug in the ground or wall which will mine out the z-level below too. You can use long channels to act as moats, to move liquids such as water and magma from one point to another, and other creative purposes. With this option it is possible to either select floor tiles or 'full' tiles. When channeled out, the floor (as well as a wall if it exists) on your layer will be removed and the tile on the layer below will be mined out. Creating a channel could be described as "strip mining".
In layman's terms: mining an area will carve out a tunnel in the rock to crawl through, whereas channelling an area will also remove the bottom of that tunnel and make a second tunnel one level below, creating a Trench of sorts.
Because it has the potential of removing two layers of wall and one of floor, channeling has the potential to be the fastest mining method. However, be careful to ensure that multiple miners working on the same channel don't undermine each other, causing them to fall through to the tile below. If this happens you may need to build a stair or a ramp in order for the fallen miner to escape.
Creating channels
Open the designation menu and select channel. Mark out the area you want to create as a channel - be careful to check the z-level below too! When building channels diagonally, a single line is enough - water, like creatures, moves in all 8 directions (+ z-axis).
The channel designation can be used to dig upwards. If a square is designated to be channeled and the z-level below has either a ramp or upward stair the miner will stand on that and remove the roof. In the same manner, designating channels over existing downward stairs will make the dwarves remove the staircase.
Designating a channel on the level underneath an existing channel is the way to dig deep pits or well shafts. Designating an adjacent up/down staircase is the easiest way to give access to the miner who will dig the shaft.
To swiftly channel out large areas multiple Z-levels deep, the fastest way is to designate the entire area as up/down staircases. After the staircases are dug out, designate the entire top Z-level for channeling, and repeat for subsequent levels once the upper ones are completely emptied. Once all areas are channeled, designate the bottom level for Remove Up Stairs. You may consider leaving a single column of stairs leading back to the surface, removing it the same way once you're done, to aid miner access; in any case, this method requires that you give your miners a way out of the bottom level once they've finished.
If the area you wish to channel is many Z-levels deep, it can be more efficient to build a construction shaped like the hole you would like to create on top of a support at the highest level. If the support is linked to a lever, and the lever is pulled, the constructions will fall through every floor until hitting solid material. Be sure to keep dwarves and valuable items clear of the falling debris.
Using channels
Creatures cannot pass over open channels but you can build floors, bridges, hatch covers, or place floor grates on top of channels. Grates will allow fluids to pass through.
To move liquids simply connect one end of the channel to the liquid source, and the other to your desired destination. You will probably want to leave digging through the last square prior to the liquid until all your floodgates etc. are finished. An unexpected flood can set your fortress back several seasons.
Be careful when tapping into a reservoir, such as the bottom of a lake, since the pressure head will cause the water to overtop the sides of the channel. Magma will not do this.