Magma is red-hot molten rock that wells up from deep within the earth (but not so deep that it cannot be found by dwarves), entering the map either by the edges or by the area beneath a magma pool. Magma that emerges aboveground is called Lava; however the substance itself remains the same. Magma is very Fun.
Magma serves as a heat source, replacing fuel in magma smelters, magma forges, magma glass furnaces, and magma kilns. Magma is extremely hot which can lead to even more Fun. Materials that can withstand the temperature of magma are called magma-safe, and the list is rather extensive.
Magma never cools, but can evaporate if left at a depth of 1/7 for long enough. When magma is mixed with water it forms obsidian (and steam). Note that magma located above semi-molten rock will be listed as a Magma Flow; magma in magma flow tiles will disappear when mixed with water (instead of cooling into obsidian).
Without screw pumps to impart pressure, magma flows rather slowly (though no more slowly than unpressurized water). A pipe to bring magma across the full map can take as much as a year to fill. This, combined with the fact that it will evaporate, can make filling a reservoir difficult and tedious. As a rule of thumb, the area coming out of a 1-wide-pipe shouldn't be more than three squares wide and 20 squares long, or else it will evaporate as fast as you fill it.
 Magma sources
Magma occurs in several different geological formations:
Although the name suggests them as pools, they are more like pipes. They can be found underground, however they rarely reach the upper z-levels (40+). Most end a few z-levels above the magma sea, though some may span more than 100 z-levels. Magma pools seem to be always connected to a magma sea, and the sea and pipe can occasionally reach up to the same level, making them hard to separate. However, magma pools can be identified by the obsidian walls which surround them. Magma pools will slowly refill themselves, giving the player an infinite source of magma. The entire embark tile containing the pool will produce sporadic bursts of magma until the magma within it is at its natural level (ie. the magma level at embark) or until it is halted by a bridge, floor, or bottom of a wall.
Volcanoes are magma pools that extend all the way to the surface. Volcanoes are an endless source of magma as they will always refill themselves. They never erupt, unlike their real-life counterparts. Volcanoes are geographical features visible on the location screen, making them much easier to find when choosing a site for your fortress.
The magma sea is a large body of magma deep under the earth. Nearly all maps will include a magma sea at the lowest z-levels, though it's inconvenient placement may inspire your dwarves to bring the magma up to the fortress proper.
 Finding magma
Nearly all maps will have magma available at the lowest z-levels, but it can be advantageous to select a site with a more easily accessible source, particularly when starting out.
Volcanoes are visible on the "local" screen in the starting location chooser. It is represented as a red ˜ mark (a double tilde) - essentially it looks like red water. Note that red ˜ marks in the "region" screen mean something different entirely (e.g. red sand).
After you have chosen to embark in a place that has a volcano, and once your dwarves have arrived at their target destination, you should see a large red pool of lava on your map. If you don't, you should expect your volcano to be somewhere underground. You then have to use exploratory mining to find it. If you can find a large patch of obsidian on the surface that is devoid of boulders, chances are there is a magma vent below, so that would be a good place to start your mining.
Much harder than simply finding a volcano is finding a volcano that is also near suitable terrain for building. Depending on your requirements - you may be looking for a source of running water, or a mountain for minerals, or a healthy tree population, a layer of flux for steel production or even all four - suitable building sites can be extremely scarce.
 Working with magma
Although magma is a liquid, it does not move via pressure unless it has been pumped. This reduced rate of flow can allow miners to survive digging into a magma reservoir, if they are lucky enough. There are ways to minimize this risk however:
Digging From Above: If you can find a suitable position above the magma, your miner can dig a channel while remaining above the level of the magma. Be warned, however, that your dwarves might take the ramp down into the magma channel as a shortcut; preemptively designating the channel for restricted traffic is recommended.
Diagonal Digging: Magma moves more slowly diagonally than orthogonally, giving the miner more time to escape. However, slower flow means you must keep in mind the evaporation. You should dig a smaller channel, wait for it to fill up, and extends the channel by Digging From Above. Workers that dig into a magma reservoir are not instantly killed as the magma touches them, but they are set on fire, which will kill them very quickly. For this reason, taking steps to ensure there is adequate water available to extinguish flaming dwarves running in random directions is advised before digging into any magma pools from the side. Channeling a single square wide pit across the planned magma pipe one tile away from the wall to breach and filling it with 2/7 water using the pond zone tool is recommended, so the panicking dwarves have no choice but to run through the water, and the water itself turns into an obsidian wall as soon as the magma flows into it.
Busy To Leave:  Dwarves who dig into magma often die not because they are unable to flee but because they choose not to. By ensuring a dwarf has another task waiting (ideally far away) they will immediately move away from the ensuing magma flow.
Simply mine up to the corner of a lava tube and then smooth the last tile. Dig a staircase within a few tiles of the place where you will be breaching that leads up and back into your fortress, this will allow your dwarf to get out before the magma gets him. Now designate the smoothed corner to be carved into a fortification. Now immediately when the dwarf begins to carve the fortification, (and this is the most important part!), designate a bunch of other tiles to be smoothed/carved. It's not important that your dwarves actually smooth, carve, or engrave those tiles, what is important is that your dwarf immediately takes another smooth/carve/engrave task elsewhere in the fortress when they finish the current one. If they do not then they will pause for the briefest of instants as they pick a new task, resulting in their death. If they have the job though, they will instantly turn and head up the staircase, stopping the magma from catching and killing them."
Exploit From Below:  Miners are able to mine out tiles diagonally above them even if there is a bridge over their heads. First you dig out your magma tunnel to feed magma to wherever in your fort you need it and dig it right up against the volcano pipe. Then you channel a trench against the pipe that can be the width of the tunnel if you wish. Build a magma-safe bridge over the trench, making sure to cover it completely, and then seal off access to the magma tunnel. Dig a new separate path to access the now bridged-over trench. Finally, designate the magma wall on the Z level of the magma tunnel for mining. Your dwarves will stand in the trench beneath the bridge but will somehow still mine out the squares diagonally above them, causing the magma to flow safely onto the bridge leaving your dwarves unscathed."
Make sure the top z-level is sealed off from miners and dig ( - ) the highlighted tiles on the upper z-level.
 Bringing Magma Up
Magma can be brought to the surface by three different methods: pump stacks, magma pistons, and minecarts. Pump stacks are conceptually the simplest, but require an enormous amount of in-game time to make. Magma pistons tend to be faster to make, but require more time to understand how to build them. Minecarts are a simple solution, but require more management than pump stacks because they can overfill a reservoir.
 Pump stacks
Pumping magma up from the magma sea via a conventional pump stack is a lot of work, requiring dozens of pumps and significant amounts of power. Making all of the pumps magma safe also requires a lot of precious materials like iron, or a functioning glass industry.
 Magma pistons
Magma pistons are another way to move magma near the surface. Magma pistons require less time and fewer precious materials to construct than pump stacks. However, magma pistons are a bit more complicated than pump stacks, so it takes more time to understand how to operate and build them.
Minecarts submerged in 7/7 magma will fill with magma. Minecarts full of magma can be tipped at track stop, which will pour the magma in a specified direction from the stop. Mine cart tracks can be built from the magma sea to the surface. Minecarts used for this must be magma safe. It may also be necessary to build rollers in magma to pull the minecarts out, which would also need to be magma safe. A minecart track can be operated by dwarves or fully automatic, using powered rollers. Depending on the placement of the track stop, dangerous overflow can be prevented by making the track stop of a material that will melt/burn once the reservoir begins to overflow.
Impulse ramps can be used to get magma-filled carts to whereever they need to go without having to provide power to an extensive system of rollers, and will fully automate bringing magma to the surface. Since such systems require comparatively little designation and magma-safe materials, work can be begun much earlier than some other methods of bringing magma to the surface.
In one design posted to the forums by gchristopher, a pump can provide power the roller, making the ramp eligible for building the roller, and keeping the trench at 7 magma so the carts fill instantly.
If you drop minecarts in directly from at least 2 z-levels above onto the right ramp, this setup has the magical property that it can handle an arbitrary number of minecarts, and dispense them at a constant controlled rate. Carts are pushed up the left ramp by the roller.
If you extend the right bridge, that tile ceases to be a ramp. Exactly one minecart will fall onto the tile and stay there, and all other minecarts dropped from above will form a quantum pile 1 z-level up.
The last time I [who?] built one, I timed the cart dispensing rate at 1 per 8 ticks. This is slow enough that carts can be brought to the surface using an impulse ramp spiral, but fast enough that you can still quickly cover a large area with magma.
The same design works with water, for giving you a lot of flexibility creating tall waterfalls without pump stacks, quickly and cheaply.
 Using magma
The primary use for magma is to
flood your fortress power magma smelters, magma glass furnaces, magma kilns, and magma forges. To power a building with magma at least one of the external eight squares must be a hole above a square of magma on the level below. Placing one of the workshop's impassable tiles above the magma conveniently prevents clumsy dwarves from falling in. Magma used for power is not consumed; a single tile of magma can operate the furnace indefinitely.
 Properties of magma
Magma behaves the same way as water with the exception of not being affected by pressure (except when being moved by a screw pump) and apparently not showing flow. Magma will turn into obsidian1 if it touches water. In the game, magma's temperature is . See the list of magma-safe materials for more information on what can (or cannot) be safely submerged in magma.
Tiles directly adjacent to magma will be heated to a temperature of
☼ when placing digging designations and causing unrevealed mining-designated tiles to cancel their designation (with a "warm stone" warning) once they are revealed.
Constructions (walls, floors, etc.) of any material can safely contain magma. Non-construction buildings (doors, bridges, pumps, etc.) that come into contact with magma should be built entirely of magma-safe materials. Non-magma-safe components will eventually melt and the building will deconstruct. Any mechanisms likely to come into contact with magma should also be made of magma-safe materials.
1 - specifically, one of the inorganic materials having the [LAVA] tag, selected randomly per biome during worldgen.
 Dangers of magma
Any contact with magma results in nearly instant immolation, followed by death if water is not close at hand. Additionally, dropping large items into magma will generate clouds of magma mist which can set your haulers on fire if you aren't careful. Magma is also home to various fiery creatures which can present a significant threat to unprepared fortresses.
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Magma is very well known for being the perfect solution to any problem encountered by dwarves. Giant badger invasion? Pour magma on it. Noble being his usual snotty, useless, arrogant self? Pour magma on it. Door locked due to invaders? Pour magma on it! Flooded your fortress with magma? Congratulations, you just won the game!
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