|This article contains massive spoilers. If you do not wish to have your game experience spoiled, do not scroll down!|
 Reaching Hell
Upon digging sufficiently deep (usually z-level ~160, but it can vary wildly) you will find huge magma lakes. These are surrounded by semi molten rock, which cannot be cleared by digging but seems otherwise stable. Eventually, you will reach a layer which contains nothing but molten rock and little to no possibilities to pass. You can reach the critical layer by digging through solid rock which gets rarer the deeper you dig. There are three ways through the impassible layer:
Digging through an adamantine vein: Adamantine veins are shaped like vertical tubes and breach through any layer of semi-molten rock, right down into Hell. The lower layers of the vein are often hollow and act as a tube, leading straight to your doom. Breaching into the hollow center of a vein has the same effect as breaching into Hell, so beware. You could easily dig around the hollow core provided you knew where it was, but every vein's hollow center is located a different height - the risk is all part of the fun. Note that adamantine veins always seem to end in mid air, so once you breached the final layer you face a horde of demons which hovers directly below your feet.
Entering a demonic fortress: These fortresses are found once per map tile, as frequent as the old eerie glowing pits. They also go right through to Hell, however they are defended by undead creatures and demons on the lower levels. See the full page for more details.
Tunneling through semi molten rock: A new method has been discovered that allows your dwarves to engineer a convoluted system involving controlled cavern collapsing and exploiting the odd behavior of a magma flow tile. Not only does it allow access to Hell, you can further exploit the Magma Flow tile to dig through Eerie Pits, duplicate any type of stone (including raw adamantine) and mine slade. The whole thing is documented and explained in this Forum Thread.
 Hell itself
You will receive the above announcement upon piercing Hell. One step later, you will get a second announcement: "Horrifying screams come from the darkness below!" At this point a massive number of demons will be spawned - though the number is not fixed, the demons will likely fill many pages on the Units screen. You'd do well to prepare for the worst.
Hell takes the shape of a large open cavern made of slade. It expands out in all directions without bound with a height varying between 1 and 4 z. Its rough floor slopes up and down, making it possible to walk around and explore should you get an adventurer down there. In the floor are glowing cracks that shine purple light up into the main cavern of Hell - whence comes this light, mortal minds dare not speculate.
Those cracks are referred to in-game as an Eerie Glowing Pit. They are, as the name suggests, bottomless pits - falling into them results in instant death, just as with standard chasms in older versions. As such, some precaution should be taken when engaging enemies near them, as they usually charge against any creature near them. Floors can be built over them, and deconstructing them will reveal the pits again.
In Hell proper, demons spawn on the edges much like animals do on the surface; demons thus spawned will be content to meander Hell without going out of their way to extinguish all life they can reach. It is yet unclear whether the number of demons is finite; in information exported from the legends screen, the population of demonic species is listed as "unnumbered," a descriptor usually reserved for ants, worms, and other omnipresent vermin.
If you dig down through a hollow adamantine pillar, but had previously blocked off the particular hallway with a drawbridge and raise it, you can discover the location of all adamantine on the map with no negatives.
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