Environment

[Mr. Cheez-It]

Walking through wild caves without end is different than stalking monsters in a dunegon near the surface.  Travelers venturing into the Underdark enter a world in which nothing can be taken for granted.  Vast portions of the Realms Below are wastelands devoid of food, water, and even light.

Light

Except for any lanterns that character bring into the caves, the Underdark is absolutely dark.  Coupled with the tomblike silence of most passages, absolute darkness can unnerve even the most experienced caver.  Thus, extra lights and oil (or magical means of illumination) are important for any cave trip.

Artificial Illumination

Halflings, humans, and other surface dwellers without darkvision must absolutely carry some form of light with them in order to navigate the Underdark.  While torches or lanterns are feasible for relatively short trips, a journey of more than a few days might require dozens of torches and oil flasks, making non-magical light impractical for long expeditions.

The best and most efficient means of providing light in the Underdark is to use minor alchemical or magical items.

Natural Illumination

While most of the Underdark is exactly that, some locales possess sources of natural illumination.

Luminescent Growths:  Rare forms of lichen, fungi, and moss are bioluminescent.  In sufficient quantities, these growths can illuminate large caverns.  Most bioluminescent growths are quite dim, so a cavern illuminated by glowing moss or fungi is typically as dark as starlight, although some particularly bright regions are equal to moonlight.

Molten Rock:  Areas with exposed pools or streams of magma are illuminated by the dim, ruddy glow of the molten rock.  The glare of molten rock is typically equal to moonlight.

Radiant Crystal:  Some rare rocks of the Underdark are naturally radiant, ranging in brightness from starlight to full daylight, although daylight equivalence is quite rare.

Reflective Stone:  While not naturally luminescent, caverns made partially of reflective stone can be much more easily illuminated by small light sources than normal.

Air

In general, the Underdark is surprisingly well ventilated.  Vast subterranean spaces and the rare planar connection to the Elemental Plane of Air provide plenty of good air for living creatures.  However, this is not universally true.

Poor Air

Sealed chambers, isolated caves, and water-isolated caverns have finite supplies of good air.  Creatures that don't breathe need not concern themselves with air quality, but other travelers do.  Any enclosed space that is reasonably airtight can quickly become stale or depleted.

Stale Air:  Regions that are poorly ventilated are not immediately deadly, but the stale air can quickly wear out even the hardiest travelers.

Depleted Air:  Depleted areas can prove deadly in a matter of minutes.

Gases and Fumes

Some areas of the Underdark are plagues by air that is not just stale, but actively toxic.  Often this situation results from geothermal activity in the vicinity of volcanoes.  Sometimes the escaping gases emerge with an audible hiss or a putrid smell, but not always.

Gases tend to concentrate in areas that are isolated in some way from the nearby passages.  For example, a passage that dips down sharply and then climbs up again forms a natural, low-lying pocket where deadly fumes can accumulate.  An air-filled passage sealed by water siphons at either end could also concentrate deadly gases.

Irritating Fumes:  These gases cause coughing, stinging of the eyes, dizziness, and similar difficulties.

Poison Gas:  These gases are deadly.  Some poisonous gases also have the characteristics of irritating fumes, so their presence is obvious.  Others, however, offer no telltale burning of the throat of stinging in the eyes to warn creatures of this danger.

Explosive Gas:  Some naturally occurring gases can explode in the presence of open flames.  If a burning torch or lit lantern is brought into a pocket of explosive gas, the vapor explodes.

Climate

The temperature underground varies depending on the depth and the presence of geothermal features, but the vast majority of the Underdark has a moderate temperature throughout the year.  It tends to be on the clammy and chilly side, but typical adventuring garb is sufficient to keep characters warm and dry.

Cold Water:  Water is almost always dangerously cold in the Underdark, since it is never warmed by sunlight.  A character who becomes soaked in cold water is much more vulnerable to cold conditions than a dry character.

Geothermal Heat:  Caverns heated by geothermal activity can be quite warm; in fact, hot or severe temperatures are common near volcanoes.  Any chamber with exposed magma is severely hot, and possibly extremely hot.

Ecology

Besides its many predatory races and monsters, the Underdark is also home to a variety of natural animals that live their lives normally below the ground.  These animals include (but are not limited to) bats, crickets (and cricket droppings and eggs), eyeless cavefish and crayfish, springfish, salamanders, rats, flies and gnats, and spiders.  Additionally, it is not unusual for creatures in the Upperdark to encounter the occasional raccoon, frog, stray dog, or even lost dwarf, elf, or other representative of a surface race who has accidentally fallen down a deep shaft or well.

Plants and Fungi

Plants of one kind or another are the beginning of any food chain.  By organizing inorganic minerals and capturing the energy of sunlight, plants create food that animals of all kinds depend on.  Since plants in the Underdark do not have access to sunlight, they must make food by other means.  Thus, most take very different forms than the green plants of the surface world.

Most of the Underdark's plant life consists of a tremendous variety of fungi.  Fungus normally requires some amount of decaying material to thrive.  So where does the fungus find its food?  The answer is simple:  magic.  The natural magical radiation of the Underdark and its various planar connections support many weird fungal growths, as well as lichens, mosses, and other simple plants, whose existence would otherwise be impossible.  In effect, faerzress is the sunlight of the Underdark, forming the basis of the subterranean food chain.  Underdark regions particularly rich is faerzress or planar energies have been known to support fantastic forests of pale, gnarled trees or crystalline plants.  These growths are completely adapted to their lightless, hostile environment.

Surprisingly, however, green plants are not entirely absent from the Underdark.  Some caverns illuminated by particularly bright radiant crystals can actually support green plants.  Caves with this sort of dazzling illumination might be filled with grass, moss, ferns, creepers, or even small trees.  Any such place is a treasure beyond price in the Underdark, and it is certain to be guarded by deadly spells, monstrous guardians, or both.

Animal Life

The Underdark supports a surprising variety of animals.  A few herbivores exist there, but most are predatory in the extreme.

Many mundane hunting animals, including bears, lions, bats, rats, and monsters of all sorts, can be found in cozy underground lairs near the surface.  In deeper places, animals must shift from the surface world ecology to the Underdark ecology.  Many surface creatures are ill suited for such shifts and therefore cannot be found more than a few hundred feet from a cave mouth, except for places where the surface ecology is somehow maintained in the Underdark.