Ignore all maps from the “Underdark” hardcover book. That book documents the Underdark in the “Forgotten Realms” campaign setting, and I only use it as a general reference to the conditions, races, and magic that are found down under on my world.
The main map used for the Underdark can be found in the “Queen of the Spiders” megamodule (originally published in D1 “Descent into the Depths of the Earth”). It is not reproduced here for obvious copyright issues.
There is no significance to the shaded area on this map. You may assume that something was spilled on the map. There is also no significance to the dark shaded hexes vs. the light shaded hexes. They are all places of interest and, for the most part, should be avoided.
The word “Underworld” is still primarily used by those in the upper world, though it represents an ignorance that confuses the underground realm with the place where certain people end up after they die. The native inhabitants more commonly use the term “Underdark” - though even this term is rarely used as it requires a comparison with the above ground world (and most denizens down under resent any link with upper world).
It should also be pointed out that the terms “Upperdark”, “Middledark”, and “Lowerdark” used in the Forgotten Realms book are never used. (Though at least one upper world lorist used those terms as if they were common).
As you can tell from the map, the Underdark consists of dozens of isolated communities spread over a few hundred square miles of interconnected tunnels. These tunnels often (but not always) correspond with water flows - the chief of which is the river Alph, which (naturally) flows down to the “sunless sea”.
Each community is essentially self-sufficient, however essential resources (primarily foodstuffs) are often constrained. To offset the meager natural supply, the undersocieties must augment their stores with magic. Thus, societies which breed spellcasters have an evolutionary advantage and most locations down under are rich with magic.
Even with magic, though, deprivation is common. This has led in most cases to societies where compassion and charity are an unacceptable luxury.
To keep these potentially destructive forces in check, a complex system of trade and diplomacy exists. Since they can coexist peacefully with nearly every race, the Xorn serve as the primary diplomats in this strange “League of Nations”.
The home of the Svirfneblin (Deep Gnomes) is called “Glimmerland” and is located in and around hex Y25 on the main map. It's location is considered an open secret, but is protected by powerful illusions and a twisting maze of trapped passageways.
Glimmerland consists of a vast cavern some 2-3 miles long and over 1 mile wide with a ceiling rising to 1000' at the center. Located at the midpoint of the cavern is a miniature sun that pulses with heat and light at a frequency of approximately 50 hours. The sun was placed by “the gods” and has been there for so long that the deep gnomes have accustomed to it entirely: sleeping for 25 hours at a time!
The ruler of the Svirfneblin is a crafty, young queen by the name of Rilla Sparkletil. Rilla is not well loved by her constituents, but masterfully holds on to her power through trickery and intimidation.
Because of Glimmerland's protections (and thorough security - including numerous espionage missions), no other groups have succeeded in a raid for over a thousand years. Also, because of their artificial sun, the Svirfneblin are a little different than most other races, so trade and other contact with the Drow, Kuo-Toa, Illithid, etc. is kept to a minimum. This isolation is reinforced by the fact that the Drow and Kuo-Toa actively hate the Svirfneblin and will kill (or sacrifice) a captured Deep Gnome rather than enslave him.
The Drow are the most common and (ostensibly) the most powerful of the underraces. Their main concentration is in the upper, left corner of the map and is known as the “Vault of the Drow”. Here lies the fortress Erelhei-Cinlu, which is the home of the most powerful Drow clan in the Underdark. Due to interclan warfare, this fortress has changed hands many times - and the current clanholder is not known.
While their society is of a chaotic evil alignment, individual Drow can be of any alignment. Naturally, they tend towards CE because of societal influences. Unlike other CE races, however, the Drow are not a smash-and-kill race. They are subtle in their evil (primarily from deep racism) and are chaotic primarily in the sense that they hate strong, central government. The natural Drow racism extends to inter-clan rivalry and while trade and negotiation between the clans is necessary for survival, it is almost always done at knife point.
Many Drow clans have left the Vault (some by force, others to seek more profitable lands). Thus, there are drow enclaves scattered throughout the Underdark (and beyond). Roughly half the marked hexes on the main map contain Drow in large numbers.
The Kuo-Toa are the next most common race, but as they live only in or near water, they do not interact with as much of the Underdark. Note that while the Shrine of the Kuo-Toa exists at map hex L2-42&43, the main concentration of these fish people is in the sunless sea (upper, right corner of map).
While they are certainly not numerous, the Illithid (Mind Flayers) are easily the third most powerful of the underraces. Their base of operations is an island in the sunless sea, but individual Illithid live and move throughout the Underdark. They are often seen as the invisible hand that guides all politics in this realm and are used as the boogeyman in bedtime stories for all denizens of the deep.
While many other races exist in smaller numbers, the Grimlocks and Duergar are the only ones with sizeable numbers.
F35 is the "Cave of the Giants" and is significant only in that it is the most common location for upworlders to enter the down below.
Q32 is known as “The Damned City” and is an ancient above ground city that was plunged below the surface a couple thousand (or more) years ago. It is now home to undead and should be avoided.