Travelling in the Underdark
Travelling in the subterranean gloom is difficult for a great number of reasons - of which darkness, danger and extreme geography are just a few. It is well-known, for example, that one of the few sounds that is nearly always present in the Underdark is the steady drip of water from a distant ceiling. The ground is often slippery and difficult to traverse; even when it is not, it is a natural cavern: strewn with rubble, random holes and irregular rock formations. In many places, tiers as tall as 3 feet must be regularly scrambled up or down.
The result of this unforgiving terrain is that movement speed is halved for those on foot - and reduced further by two-thirds if travelling in darkness with no means of seeing. There are ways of mitigating this, of course. Mules and other surefooted pack animals are not hindered by these conditions; there are also some rare mounts, such as the giant lizards ridden by drow, that can be used to traverse the Underdark swiftly.
The height of the ceiling in primary tunnels and varies from 20 to 50 feet, and the roofs of larger open caverns can rise as high as 100 feet. In secondary tunnels, it is usually between 10 and 40 feet in height. Smaller side tunnels or secret passage ranges from 3 to 20 feet tall.
Camping for the night in a passageway (including a spur of room off the same) incurs a 1 in 12 chance of encountering a wandering monster; large primary "arteries" increase this chance to 1 in 10. The same check is made for each mile that is travelled by the party. Large open caverns have a 1 in 10 chance of an encounter, just like primary passages - sound carries far in open spaces under the earth.
The Limits of Infravision
Generally speaking, infravisual range is a set distance - usually 60 or 120 feet. In most parts of the Underdark, this is more than sufficient. There are places, however, where it is not - such as the particularly large caverns where drow cities are built. In places such as these, ways have been devised to improve visibility without resorting to light - as well as ways to reduce visibility to infravision without requiring an Invisibility spell.
Sources of bright or distributed heat are the best way to improve infravision. Even if your infravision only extends to 120 feet, a candle burning in the distance will glow like a fireball to a drow. Furthermore, the endless expanse of cool stone, which appears as a uniform bluish grey, makes smaller heat signatures much easier to discern to those who are practiced at doing so. Those who have spent a month or more in the Underdark can "see" beyond the maximum range of their infravision, albeit in a blurry and low-fidelity way. You may make an attack against a distant creature that you can just about make out with infravision, but they are considered to be invisible (imposing a -4 penalty on attack rolls) unless they are standing near a heat source that "outlines" them.
Infravision & Stealth
A thief with the ability to Hide in Shadows can hide from infravision, but it must be kept in mind that hiding from infravision requires a different approach to hiding from the naked eye. Instead of a curtain, shadowy corner or a bush, a thick stone wall (which can hide your heat signature) or a thermal vent (which will cloak you with hot air) are examples of places you might hide from creatures with infrasight.
Some means have, of course, been devised to make hiding one's heat signature easier. The cheapest and most common solution for drow soldiers is the so-called "hot coat": various plates of heavy iron and belts of heated leather arrayed at odd angles. Although the ensemble takes a turn to prepare and only lasts for an hour before the leather cools and metal warms up, they serve as camouflage. This has the effect breaking up the profile and adds a 5% bonus to HiS attempts or a -1 penalty to enemy surprise rolls.
There is also a magical option that allows you to hide yourself without having to resort to an Invisibility spell. So-called "rock powder" is a fine powder made by drow alchemists, which is manufactured from the dust of Faerzress Moths. When sprinkled over a person, it will radiate a strong aura of cold for the next hour. Although the cold has the effect of imposing a -1 penalty to Dexterity checks and attack rolls, it has the effect of making the subject indistinguishable from the stone walls of the Underdark. This effect confers 99% HiS when standing still in front of a cold stone wall with no sources of light or heat nearby. The rest of the time, it gives a 15% bonus to HiS or a -3 penalty to enemy surprise rolls.
Finally, infravision makes tracking much easier. Warm-blooded living creatures leave a "heat trail" on the cold floor of the Underdark whereever they walk. Although it is faint and fades quickly, this heat trail means that you can treat the ground as "soft or muddy" for the sake of the Tracking NWP. However, there is a -1 penalty applied per turn since the trail was made, rather than every 12 hours; the heat fades quickly.
Much has been made of the strange magical field of radiation that separates the Underdark from the world above. Some believe it is simply a natural phenomenon created by certain minerals deep beneath the earth, but those with knowledge of the planes will note that a great majority of worlds in the Material Plane have some form of Underdark, and that the Faerzress is universal.
In truth, the Faerzress owes its origins to the fluid nature of the planes. While to the inhabitants of Morus, their world may seem to be discrete and wholly separate from rumored "otherworlds", in truth there is a great deal of bleed-over between planes. Deep oceans open into Elemental Water; the heart of a volcano opens into Elemental Fire. The Underdark is the same: the deeper one goes, the closer one becomes to Elemental Earth. This is where the Faerzress comes from - it is a natural result of the boundary between planes.
Effects on Magic
Perhaps most well-known are the effects that the Faerzress has on magic, which are a direct result of its interplanar nature. Spells of conjuration or divination cannot be cast reliably either through or within the Faerzress. This means you can neither teleport into the Underdark, out of it, or within it.
Another, lesser known effect is on spells that produce bright illumination - such as Light or Continual Light. Instead of brilliant illumination, these spells instead produce soft, ale-colored pools of dim illumination. The drow hate even this level of light, of course, but it will not affect their combat abilities.
Effects on Minerals
The Faerzress has effects on natural materials that are well-documented. Ordinary minerals interact with the radiation in strange ways, causing them to take on wondrous properties. Unfortunately (or perhaps fortunately for surface-dwellers), these magic properties invariably carry the drawback of immediate disinteagration on contact with sunlight and slow degradation if removed from the Faerzress.
The most well-known Faerzress mineral is adamantine, the almost unbreakable variant of mithral famed for its incredible ability to hold enchantments.