Nearly 200 kilometres below the earth's surface are these dungeons which are much larger than those encountered on Level 3 and are closer in scale to a small city. All of these will definitely be inhabited even if not by the original owners.

At 700 kilometres below the surface is the first level of true civilization, such as it is. These are the cities (or what remains of them) of the various subterranean races. The map shows a number of connecting tunnels between the major inhabited regions of the Middle Depths. All connecting passages are large enough to be passable to human-sized characters. Other factors, such as vertical obstacles or water currents are handled on an individual basis in the area description, or can be generated randomly by the GM. The black lines represent the tunnel underways (highways) running deep underground. The countries are shown only as reference for where the underground cities are with respect to the world above ground.

Because most of the Underground has been only cursorily surveyed, and given that the largest caves are only a few miles long, few geographical features would be considered noteworthy to a surface dweller. Lakes tend to be the largest features, although the nearby stone may dip below the surface of the water, breaking it into quasi-separate regions that can only be identified as the same body of water by the most meticulous cartographer. Tunnels in the Underground extend for miles, some ballooning into caverns thousands of feet across, only to shrink to narrow spaces too small for a Halfling to squeeze through. The largest cavern halls become representations of the surface in miniature, with hills, valleys, underground rivers, and lakes. In this three-dimensional environment, most races make use of the walls and ceilings of their caverns, accessing the higher levels with natural or magical flight or levitation or wall-crawling mounts such as giant spiders or certain breeds of lizards.

The name ‘Underground Highway’ doesn’t do justice to this vast network of subterranean passageways. Each stretch of the Underway is unique, with its own peculiar properties. If there is anything experienced explorers have learned, it is how much they have still to learn about the deep world. The public and secret passages of the Underway are well known and easy to travel. These are what appear on the map below. The Underway is a marvellous place and provides ways to travel long distances quickly or avoid certain hazards from the surface, but a wise traveller learns as much of the road ahead as he can.

Next, travellers quickly discover that navigating underground isn’t as easy as on the surface. Awareness of the surroundings can be a great help in underground navigation. Having some knowledge of geology is most valuable to
interpret changes in the rock walls in relation to where the various exits of a cavern lead. Smooth walls indicate the presence of water and mosaic multi-layered walls are evidence of the forces that were at work in the distant past. The same goes for animal and plant lore. The Underway plays tricks with the mind. Not only the darkness, but also being confined to narrow enclosed spaces, the many slopes and turns that make scouting ahead impossible, the need to be constantly aware of your footing and the stability of the roof have driven many explorers to, and sometimes over, the edge of madness. Even experienced travellers admit that this can be a tense experience occasionally, but they also
tell about the peaceful silence and reaching a meditative state walking through dim, quiet tunnels.

The blue lines represent all the various main river tunnels flowing through the vast dark reaches of the fifth level of the Underground. These are on the same level as the cities. All sections of underground waterways are navigable by boat, assuming a skilled pilot and a little luck. There are also much smaller tributaries not displayed here.

To determine the width of a section of waterway, roll D6 and multiply the result by 5, for a possible spread of 5-30 metres. The width remains constant, with minor fluctuations, for one kilometre, after which you must roll another D6. On a 2-5, the river remains at approximately the same width. If the result is 1, the river decreases in width by 3 metres. If the result is a 6, the river expands its width by 3 metres, even if this makes it wider than 30 metres. The water in underground rivers is usually numbingly cold. To check the temperature of a given area, roll D6. On a 1-5, the river is very cold. On a 6, some heat is reaching the water. To determine how warm the water is, roll another D6. On a 1, the water is cool, but slightly warmer than normal. On a 6, it is boiling hot. A result ranging from 2-5 means a degree of warmth proportional to the roll. Only water that is boiling hot is unsafe for unprotected characters to swim or fall into.

Underground rivers tend to be swift, violent, and tortuous in their windings. It is a rare river indeed that flows level and smooth for more than a few miles at a time before disappearing into a deep gorge or sinkhole in a fuming waterfall. Rivers are the great builders of the Underground, the natural force that sculpts great caverns and brings life-giving energy and food to sustain the Underground ecology. Most rivers are surrounded by a halo of living caves, which can be valuable real estate indeed.