Underdark

The following is quoted from one of my D&D source books/modules (possibly Vault of the Drow or something). Additional information about the Underdark has been acquired from Forgotten Realms Helps

There are faults, spurs, splits, rubble, small cave areas, sink holes, crevasses, columns, stalactites, and stalagmites. Most areas are damp and dripping. There are occasional pools, rivulets, even streams. The rock is of all colors, although grays, browns and yellowish tan are common. Parts of the routes are worked, but much of the tunnel system will be natural passages, caves, galleries, etc. The route always descends.

There are occasional phosphorescent lichens and swarms of fire beetles now and again. Should the party ever remain still and listen, they will note many strange sounds, twittering, squeaking, scrabbling, clatters and clumps.

Now and again a current of dank, cold air can be felt moving downwards and bearing a musty scent throughout the corridors of this dismal underworld.

Travel along the system of subterranean passages will be at a slow rate because of the slippery and often slanting or terraced floors. Footing is treacherous in places, and the tiers to clamber down are often nearly 3' from top to bottom. The rubble and detritus, natural projections, protrusions, ledges and stone "icicles" are more common and prove a real hazard in the secondary and tertiary tunnels. These lesser traveled ways offer more places for lurking foes than do the primary arteries of the underworld.

You may have crevasses and sinkholes as often as is illustrated with the tunnel pieces, or you may include such hazards every mile or so. There is certainly a chance for a slip and fall into a crevasse if proper care is not exercised by the party. Crevasses are 110'-300' deep, so consider any fall as fatal if it is not prevented in some way. Adventurers roped together, using poles to hold onto, etc. are not likely to slip. Sink holes are 10'-80' deep. While there is no real chance of slipping into them, if the party is running in hast or traveling without light, treat sink holes as pits.

Ceiling height in the primary tunnels ranges from 20' to 50', the average being about 35'. in the secondary passages the roof overhead varies from 15' to 40' with 25' being the average. In the tertiary tunnels the ceiling is from 8' to 25' high, with an average of 15' or so. Where enlargements occur, the roof will be near maximum height.