Underdark

World Above

The surface area of Toril. The name those who lived in the Underdark gave the surface.

Upperdark

The first 3 miles (4.8 kilometers) below the surface. It was here where the surface dwellers and those in the Underdark most often met.

Middledark

Located 3​ to ​10 miles (4.8​ to ​16 kilometers) below the surface, this layer was where most of the Underdark cities were located.

Lowerdark

10 miles (16 kilometers) or more below the surface, the Lowerdark was where even those who knew the Underdark were loath to go.

Domains

The Underdark was not one giant cavern under Faerûn, but rather, many huge networks of caverns and caves. As a result, it was not always possible to travel from one end of the Underdark to the other. The Underdark was divided into several domains that were similar to continents of the world above. While it was possible to travel from one place to another within a domain, separate domains tended to have very few passages linking them. The major domains of the Underdark were the Buried Realms, the Darklands, the Deep Wastes, the Earthroot, the Glimmersea, Great Bhaerynden, the Northdark, and Old Shanatar.

Topography

The Underdark's underground landscape varied from small subterranean fissures almost impossible for adventurers to crawl through, to wide expanses of deep water that hid coral caves, hollowed out sections of ice in which creatures lived, and places where fungus, bones, or even pure force formed "caves."

This varied landscape was as dynamic as the surface of Abeir-Toril. Some areas were always changing over time because of underground magma rivers or the emptying of underground lakes due to cracks in the rock formed by earthquakes. This meant maps of the Underdark were highly unreliable.

Environment

The Underdark's environment varied vastly from the surface world, making adventuring through it a dangerous expedition. Just like trekking through a desert, adventurers traversing the Underdark's passages needed to be well prepared.

The Underdark was mainly without any source of light save for the occasional luminous rocks or fungi. Large portions of it were also without food or water, making well-stocked food supplies a must-have when exploring uncharted terrain underground.

Lack of light combined with a tomb-like silence made traveling something only for the strong-hearted. The nerve-wracking silence made even experienced adventurers grow agitated after a few days.

Just like deep dungeons, the air could grow stale and even poisonous in the Underdark. It could contain explosive gases from underground volcanic activity or other deadly gases bubbling out of deep fissures in the rock. Bad air and poisonous fumes could collect in portions of tunnels and caves more or less isolated from the main vaults and caverns. Siphon-like tunnels holding water could also trap these gases for a long time.

Climate

The temperature underground varied according to depth and the proximity of thermal activities in the area. Throughout the year, the temperature remained stable and moderate in most regions, usually slightly clammy or chilly. The water underground was extremely cold and posed a hypothermia risk.

Plant life and animal life can be as varied underground as on the surface. Ranging from small insects to hook horrors or shadow dragons, which set up their realms deep down, everything can be expected. Plant life is unable to utilize sunlight as a source for energy, and thus takes on strange forms adapted to life underground. Many fungi are also found underground. Being unable to digest normal nutrients most of the time, they rely on absorbing Faerzress for their energy. Some dangerous flora of the Underdark were the exploding Bibberbangs.

The Underdark is extremely dangerous, especially to those not native to it. There are the usual dangers associated with caverns: claustrophobia, occasional poor air circulation, and getting lost. There is no light except for occasional patches of fluorescent fungi; most Underdark inhabitants either have highly developed senses other than sight or have developed Darkvision. Food can be extremely difficult to find, and much of the natural vegetation is poisonous. In addition to those dangers, the Underdark is also the home of many predators and often-hostile races, including Aboleth, Beholders, Cloakers, Derro, Drow, Duergar, Dwarves, Illithids, Kuo-toa, Driders, and Svirfneblin.

The Underdark follows the practices of its main driver in commerce, Menzoberranyr. Menzoberranyr accept all surface coinage, but prefer metal or gems of intrinsic value. a Drow is happier with silver or gold coins than copper pieces, because the metal can be melted and reshaped for use where the coinage is not honored. The moneylenders of Menzoberranzan also deal in trade bars, rare shells, the jarred and jellied eyeballs used by some mind flayers, all known types of gems, perfumes, and even rare foods and plants. The more exotic the currency or collateral, the higher their rates.

Commerce

Commerce in Menzoberranzan consists of two separate but intertwined systems of exchange, each specific to a certain social and political caste. 

Currency

When someone procures goods or services from a Drow of low station, such as when a house scion buys something from a vendor at the Bazaar, the transaction is carried out by the use of various coinage of silver, gold, and other precious metals. The city has no universal currency. Instead, each of the noble houses of the Ruling Council mints its own coins. The prevalence of a particular currency in the city is a good indicator of the rank of the house that minted it. The currency an individual uses is a display of his or her affiliation; money centers do brisk business as alliances shift. The merchants of the city do not turn away any currency, no matter its source. Foreign coins are assigned value based on their weight and purity.

Favors

The second type of commerce, common among noble Drow, is the exchange of favors. This system creates a web of debts and obligations that binds one drow to another. For a drow, having too many obligations hanging over one's head is tantamount to social suicide. While she is indebted, a Drow cannot afford to directly challenge anyone to whom she owes a great favor, for fear of that favor being called in and ruining one of her schemes. Despite their selfish nature, few Drow would ever dare ignore a legitimate debt, out of fear that no one would do business with them (or worse, their house) in the future. Such a stigma would quickly be followed by a loss of favor with Lolth, leading to rival houses calling for the destruction of the offending faction. 

Merchant Clans

Menzoberranzan is chock full of merchants: house agents, traders, and the more numerous independent commoners. These entrepreneurs, (particularly those who travel the Underdark to other cities and trade sites) keep Menzoberranzan vital, important, and rich. Traders and crafters often band together to form companies or fellowships. In some respects, the larger merchant clans serve as a counterbalance to the haughty noble houses, but in truth, most of these organizations serve at the whim of the Ruling Council and can be dismantled at a moment's notice. Noble houses most likely to ally with a merchant clan include House Druu'giir, House Hunzrin, and House Mizzrym. Merchant careers in Menzoberranzan are dangerous and often short. To anger a matron or even a proud young house noble is to court death or disaster to one's livelihood. The danger of this lifestyle, coupled with the restlessness of many merchants (who at heart are happiest when traveling), makes turnover high among the city's mercantile roster. The most prominent merchant clans in the city are described below. 

Black Claw Mercantile: Once the most extensive and independent merchant fellowship in the city, the Black Claw soon came under the sponsorship, then the direct influence, of House Baenre. Over time, Baenre's control became an open secret, prompting rival houses to sponsor their own fellowship, the Brown Mushroom. The Black Claw has a virtual monopoly on draw goods flowing in and out of Mantol-Derith.

Brown Mushroom Coster: Funded by a cadre of noble houses, including Barrison Dei'Armgo, Fey-Branche, and Hunzrin, this clan specializes in high-risk trade between Menzoberranzan and distant draw enclaves throughout the Underdark. Three Black Rings: This lean, well-organized merchant company is unusual because it employs creatures of almost any race as laborers and caravan guards. The clan invests much of its profits into seeking new, alternate routes through the Underdark; selling maps of these routes and hiring out guides to other merchants has become a large part of its business. 

High-Handed Fellowship: The High-Handed Fellowship is the primary importer of exotic foodstuffs from the surface world. Through its secret network of contacts, this clan keeps the larders of the matron mothers stocked with rare culinary treats from realms across Faerun and beyond. Delicacies include blood wine from Aglarond and chocolate wafers from remote Lopango across the Trackless Sea. 

Xalyth's Company: Menzoberranzan's merchants are chiefly Drow males, but Mistress Xalyth enjoys playing against gender stereotypes. She rose from humble origins to dominate gem-dealing in the city. She is known as a fair appraiser (and an under-the-table fence of stolen goods), as well as a hard bargainer who always buys low and sells high. It helps that she has little competition, because she is the only steady source of spell components in the city. 

Moneylending

Most ambitious drow run short of coin at one time or another. Some are in no position to successfully steal or extort funds, murder and rob, or gain a loan from a patron or House superior. They turn to the moneylenders, who also act as money changers for outsiders. Menzoberranyr accept all surface coinages, but prefer metal or gems of intrinsic value. A Drow is happier with silver or gold coins than copper pieces, because the metal can be melted and reshaped for use where the coinage is not honored. The moneylenders of Menzoberranzan also deal in trade bars, rare shells, the jarred and jellied eyeballs used by some mind flayers, all known types of gems, perfumes, and even rare foods and plants. The more exotic the currency or collateral, the higher their rates. A standard loan to a Drow of the city is typically in coinage, at an interest rate of 10%, compounded every 20 days. Collateral of written title to items worth 25 gp or 75% of the loan, whichever is higher, is usually required. To a visiting Drow, the standard is 12%, compounded every 40 days, but collateral equal in value to 100% or more of the loan in portable property (or a deed to land or a building in the city) must be left with the lender. To a non-drow citizen or long-term resident of Menzoberranzan, the terms are the same as for visiting Drow, but the rate is 14% or more, compounded every 20 days. Visiting non-drow are also lent money on these terms, but at 16% or more. High-risk loans (to merchants who must travel the Underdark before they can see profit enough to repay, a fugitive from Drow justice, or someone widely known around the city to be in trouble with a powerful Drow House) typically are at 20% to 25% or more, and require collateral left in the lenders possession, to the value of 150% of the loan. Most moneylenders have magic and hired swords and spies to back up their demands and defend their wealth. They are wily, ruthless, and often adopt a powerful House as patron (giving its members cut rates and ready credit), to gain its protection when a noble debtor of another House decides, as some of the younger and more reckless inevitably do, that having a bit of sport and wiping out fastbuilding debts with a quick swordthrust is easier than paying up.