An aquifer is a subterranean layer of groundwater-bearing rock or soil. Attempts to mine through the layer will result in the mined-out squares immediately filling with water, effectively halting excavation at or below the aquifer level. This, in conjunction with the fact that they are often located in areas rich in loam, and sand, makes it difficult to find great quantities of stone in areas with aquifers, making for more challenging gameplay.
Aquifers can't be drained - the groundwater is limitless. In stone aquifer layers, smoothing the walls will stop the production of water. Aquifers do not only produce water - an aquifer tile will absorb any amount of pressurized water from neighboring tiles (effectively draining all layers above the aquifer). As with water production, this ability will not be disabled no matter how much water it absorbs.
 Where they are found
Aquifers appear based on the elevation of the terrain. Low elevations, particularly those near rivers and oceans are more prone to having an aquifer present, while locations closer to mountains are much less likely, but still possible.
Layers which CAN contain aquifers:
- sandy clay loam
- silty clay loam
- sandy loam
- silt loam
- loamy sand
- yellow sand
- white sand
- black sand
- red sand
- pelagic clay
- calcareous ooze
- siliceous ooze
Layers which CAN'T contain aquifers, despite their names suggesting otherwise:
 What they do
Aquifers are tiles which produce water in their neighboring tiles -- north, south, east, west, and below. They do not produce water in the tile above them, or any diagonal tiles. Note that smoothed, mined, or channeled aquifer tiles no longer produce water.
If you are digging an up/down staircase in the downward direction, and you hit an aquifer, the aquifer tile will be revealed as damp soil or stone, and the digging job will be un-designated for that tile. If you are mining horizontally, you will similarly be warned of a "damp stone" before breaching the aquifer.
If you are digging an up/down staircase in the upward direction, and you hit an aquifer from below, the aquifer tile will immediately start producing water in the stairwell, thus leading to a lot of Fun.
 Dealing with aquifers
 Probing an aquifer
You can discover what layer lies below an aquifer layer by digging up/down stairs into the aquifer. This will reveal the tile below the aquifer layer, and if this is non aquifer (for example, clay, ore or bedrock) then you know the aquifer is only 1z deep at that location. This method can only be used to determine whether the aquifer is 1 layer deep, or multiple layers deep, but this is enough to help plan how to penetrate it. Using a pump-based method is highly recommended for multiple layer aquifers.
 Going around
If your embark site is covered by multiple biomes, there is a chance the aquifer is not present in every biome. In some maps this may be indicated by an outcropping of stone in a landscape otherwise composed of soil; in other maps the change in biome might be visible as a change in soil type or vegetation type or density. You might be able to dig down through a biome that doesn't have an aquifer, to a Z-level below the aquifer, and then (if you wish) tunnel beneath the aquifer to the previously inaccessible region.
Even if all the biomes of your site contain aquifers, they might not all be at the same Z-level. So you still might be able to dig down in one biome, reaching a Z-level beneath the aquifer in another biome.
Additionally, if your biome contains deep cliffs, for instance in the form of a river gorge, it may be possible to build a staircase down the side of the gorge past the aquifer.
 The double slit method
This is one of the most commonly-used methods, due to its convenience and power. It was originally developed by QuantumMenace, and is also mentioned below under the pump method.
 The ore method
On maps where the aquifer is not held in a layer of soil, but instead is held in a sedimentary layer such as sandstone, it may be possible to tunnel down through deposits of ore such as magnetite. For this to work you have to find a spot where there is coincidentally an ore deposit on each Z-level you need to dig through. This is only possible through tiresome trial and error, or through the use of a utility like DFHack's reveal. The trial and error method can be accomplished somewhat more easily by digging up/down stairs to reveal the layer underneath them without actually digging into the underlying layer. This method is more complicated with aquifers located in layers of conglomerate, as large clusters of puddingstone will support the aquifer and thus cannot be used to provide a path through it.
 The cave-in method
Conceptually this method involves removing the aquifer-bearing sand, soil or rock using channeling, and then dropping an island of dry sand, soil or clay into the resulting pond, a staircase can then be dug through the center of the resulting artificial island. This requires at least 2 natural dry layers.
This does not work with constructed walls since they deconstruct on cave-in.
Note: There is a bug that may prevent this method from working, collapsed layers may turn into the aquifer layer type that was dug out at that level. So, for example: We have three layers, layer 1 has the caving in section, and is not an aquifer. Layer 2 is a dug out layer that is also not a aquifer. And Layer 3 which is dug out and is an aquifer. Now, the bug, say layer 1's cave-in section lands on layer 3's dug out area, sometimes layer 1's caved in section may change into layer 3's soil type. Making it an aquifer too. Thus making the cave-in method impossible for that area.
 Cave-In Example
- Dig stairs down to the aquifer. Dig over the aquifer layer but under your "plug". You'll need a 5x5 landmass. (Slide 2)
- Channel out the area the plug will fall into. (Slide 3)
- Leave a single floor tile on top of the plug and dig out the outer layer of your plug. The plug should be a 3x3 landmass now. The single floor tile must keep the plug from falling. (Slide 3)
- Channel out the floor tile holding up the plug. (Slides 4 & 5)
- Construct floor tiles to reach the plug and dig through the middle to get under the aquifer. (Slide 6)
 Cave-In Method for Multiple Layers
If you build many rings inside one another in your top drop layer, you can breach multi-level aquifers with as little as 2 natural layers of dry soil above it. Drop the rings from the outside to the inside using constructed arms to hold the center rings in place. Once a ring drops into the water below it, pump out the water in the center and dig down another layer. When that is complete, drop the next ring and continue the process until you are through. Since you start dropping rings from the outside it is necessary to know how many levels deep the aquifer is before you begin.
Tutorial for more than one Aquifier can be found here: User:Rhenaya/HowtoDualAquifer
A more detailed example with pics can be found here: http://www.bay12forums.com/smf/index.php?topic=108340.0
 The pump method
The pumping method uses one or more pumps to keep an area dry long enough to smooth or wall off the sides, stopping the flow of water. It requires no special environment or resources, other than wood and dwarves (and patience). Most commonly, a modestly-sized section of the aquifer layer is channeled out and several screw pumps are built facing it. Directly behind each of the screw pumps a few tiles are channeled out to receive and dispose of the pumped water. When the pumps are activated, they should pump water faster than the aquifer can produce it, allowing masons to smooth or build walls around your future staircase. You will get job cancellations during this process, as stray 2/7's of water interrupt the building process. Just unsuspend the construction when this happens, as long a dwarf manages to touch the wall before canceling, it will move incrementally toward completion and eventually finish. Depending on the availability of screw pumps and dwarves, you may need to wall off one corner or side at a time, then move the pumps and repeat. When drilling through more than one aquifer layer, be sure to leave yourself enough room to build additional layers of pumps and water disposal channels on lower levels.
Things to consider:
- Flowing water will cause parents to drop their infants, leading to job cancellations and occasionally fun.
- The smaller your work area, the less water your dwarves will have to remove and the faster construction will finish. For a single-layer soil aquifer, you only need to mine five tiles (your stairway and walls directly North, South, East, and West of it); single-layer stone aquifers require only a single tile be channeled.
- Mechanical power may come in handy, but dwarf power works just fine and is much more portable.
- Channels can sometimes be used in place of walls, causing water produced by by the aquifer on one level to immediately fall and be consumed by the aquifer on the level below.
- This method may take a while.
- Aquifers do not create water in diagonal tiles, but do create water in hollow tiles directly below them. Therefore, you will want to dig two z-levels below the lowest aquifer layer before continuing with your fortress.
 Specific pump methods in detail
Earlier in the same thread, Hans Lemurson laid out a very dwarfy method that can also pierce aquifers of any depth, using several pumps and machinery. Find it here.
 The freezing method
If you are playing in a freezing or very cold landscape, where it snows in winter and instantly freezes water on the map, you can dig out a 3x3 hole in the ground using channels, and make it deeper and deeper until you reach the aquifer level. Once you reach the damp rock, tunnel into it with up/down staircases, then channel out the downstairs, the exposed water will turn to ice, digging the up/downstairs before channeling allows the tiles to safely fill with 7/7 water before being frozen, this avoids the hazard of miners being encased in ice and avoids a bug(?) where frozen water which is less than 7/7 deep does not produce a floor above it. The central square of the 3x3 hole should be tunnelable ice, so you can get to the rock beneath.
If your fortress is in a zone that gets warm, build walls around the inside of the hole to stop the water coming in once the ice melts. In order to build a wall around a 1x1 staircase it will be necessary to have a 5x5 hole, since you need to leave an outer ring of ice to seal the aquifer.
If the aquifer is multiple layers deep you will need to start with a sufficiently large hole to account for both an ice wall to seal the aquifer and a constructed wall to seal the ice wall for each layer of the aquifer. A pump based method might be preferable.
 The magma/obsidian method
If you have access to a supply of magma, you can create your own obsidian caissons. By channeling into the aquifer layer and then filling these channels with magma, or by digging staircases and pouring magma down the staircases, it is possible to create a wall of obsidian between your working area and the water-bearing rock or soil. However, changes to world generation with the last version have made this method more difficult than it once was, as it is now harder to find magma vents that extend above the aquifer level.
 The drainage method
Having made an initial hole in the aquifer, you may wish to punch another larger hole through, say for example to grow wild strawberries in the caverns. Or you may simply want an additional (natural stone!) staircase. Once you have access from below this is much easier than digging from above, and it has the additional benefit of producing a shaft of exactly the size you want.
Locate the caverns and dig a drainage shaft of up/down stairs or downward stairs up from the caverns to the aquifer (downward stairs function as grates and are far safer than channeling). Once the drainage shaft is complete punch the shaft up through the aquifer (using up/down stairs) until you hit dry dirt. Now mine out the walls around the shaft and build constructed walls to seal the aquifer. It's even faster if the walls are channeled out instead, constructed walls can be built in the open space and water falls straight through, thus construction can always be started and is never suspended. Always build the walls from the highest layer down, so the dwarves aren't having water dumped on them from above.
This method can be used to create arbitrarily large (and shaped) holes. Large holes, which would be impractical to dig from above, are very easy using this technique. It's also extremely useful for digging straight shafts through "layercake" aquifers where aquifer tiles and non-aquifer tiles are intermixed.
Just be very aware that your framerate is bound to suffer, if you are not fast with plugging the aquifer walls.
 The Chicken Run Technique
An extremely risky variation of the above is to have a reasonably fast/skilled miner dig a set of up/down staircases faster than the water from the aquifer can actually fall down the stairs and block movement into the mining tile.
If you can hit the caverns, you can drain the water out into the caverns, and build walls into the aquifer as per the drainage method.
If you fail to mine fast enough, or if you miss the caverns, your miner might be able to simply walk up the stairs to safety, and you can just try another shaft, or your miner might
drown have fun.
 The modding method
By editing the raws and removing the [AQUIFER] tag from all of the appropriate entries in inorganic_stone_layer.txt, inorganic_stone_mineral.txt, and inorganic_stone_soil.txt it is possible to remove all aquifers from the world. This can be done before creating a new world or after, if you find a particularly neat location ruined only by the presence of an aquifer. In order to modify an existing world, you must delete the [AQUIFER] tag from the raws in the savegame's folder.
 Command-line (Linux)
cd df_linux/raw/objects/ sed -i 's/\[AQUIFER\]/(AQUIFER)/g' inorganic_stone_*.txt
and generate world. To edit an already generated world, run the
sed command in the df_linux/data/save/regionNN/raw/objects folder instead. If you want to restore the tags later, you can do it with the command:
sed -i 's/(AQUIFER)/[AQUIFER]/g' inorganic_stone_*.txt
 Text editor (All operating systems)
Find the files in Dwarf Fortress/raw/objects (new world) or Dwarf Fortress/data/save/regionNN/raw/objects (already saved world).
Open the three files with a text editor (e.g. Notepad). (inorganic_stone_layer.txt, inorganic_stone_mineral.txt, and inorganic_stone_soil.txt)
Use Edit->Replace, and replace [AQUIFER] with (AQUIFER). (Use 'Replace All').
To restore the tags later, do the same in reverse. (Replacing (AQUIFER) with [AQUIFER]).
 Benefits of aquifers
While annoying, aquifers can be useful for building a self-sufficient fortress, and for water-related megaprojects. Since an aquifer can absorb an infinite amount of water, it can function as a drain for anything above it. For instance, digging a pit in a lower Z level of an aquifer, then connecting it to a breached aquifer a level above through a channel dug a level above that will create a permanently flowing, compact, secure water/power source completely contained within the fortress.
Aquifers outside ocean biomes also contain fresh water. Since aquifers are almost always located close to the surface, freshwater aquifers can easily be turned into a source of infinite, secure, non-freezing drinking water for your dwarves, eliminating the need for a cistern. While both of these roles can also be filled by cavern features, an aquifer allows you to get the same advantages without exposing yourself to potentially dangerous cavern creatures.
- QuantumMenace's two-slit method for breaching aquifers of any depth - Illustrated guide
|World Generation (Basic / Advanced / Rejections)|
|Regions - Climate - Surroundings - Map legend|
|Civilizations - Sites ( Cave - Town - Fortress - Ruin ) - Calendar|
|Aquifer - Brook - Island - Tunnel - Volcano - Waterfall|
|Biomes||Chasm/Caverns - Desert - Forest - Glacier - Grassland - Lake - Mountain - Murky pool - Ocean - River - Savanna - Shrubland - Tundra - Wetland|