Dungeons are familiar to nearly all adventurers. Some dungeons are constructed entirely from caves, while others use caves as entrances or emergency exits only. In any case, intelligent creatures have built each dungeon to serve a particular purpose. Dungeon passages generally run within a few hundred feet of the surface, although this is not a hard and fast rule. The deeper the dungeon, however, the harder the task of moving the excavated material out of the way. Dungeons are generally excavated from a soft rock such as sandstone or limestone. These materials are not as hard to excavate as granite, for instance, and still provide a solid and sturdy support for the dungeon’s corridors and rooms.
Some dungeons are excavated from dirt, and require constant shoring up (at least at 10-foot intervals) in order to prevent cave-ins. A dungeon excavated from dirt will not last as long as a stone dungeon. Some dungeons begin as buildings whose ruins are later buried by new buildings, until the original layer seems to be well underground. Dungeons can be encountered long after their construction, and may in fact be well on the way toward collapse-or they may be newly excavated or even in the process of construction when characters encounter them. Dungeons are created for a wide variety of purposes. The most common include service as jails, hiding places for treasures, lairs for bands of underground creatures, or positions of defense. In a world where flying creatures are ever-present, dungeons can prove to be stronger fortresses than castles!
Mines in a fantasy world are almost always tunnels or shafts. Some mines, particularly those developed over many centuries by diligent excavators such as dwarves or gnomes, can become complicated networks of tunnels extending through three dimensions and stretching for dozens of miles. Mines can be either active or inactive, which indicates whether or not they are currently in use. The miners who created an inactive mine might have ceased their work because the vein of material they were excavating was depleted, but this is not necessarily so. Disasters or conflicts might have forced the operation to discontinue. Active mines are usually well-guarded, and inactive mines often become the lairs of subterranean monsters.
Burrows are generally long tunnels, perhaps with a widened spot at some point, created by creatures as shelters. Only very large burrows are big enough for characters to enter; unfortunately, very large burrows are usually created by very large creatures that do not always react kindly to the intrusion. Burrows are generally excavated from dirt, but certain creatures (such as umber hulks) can create burrows in solid stone. A burrow is generally no bigger in diameter than the creature who created it, although a larger chamber is often excavated at the burrow’s deepest end. Realms are vast underground reaches made up of caverns, dungeons, and lofty passages of questionable origin deep beneath the earth.
Realms are rarely found within a mile of the surface and often extend far deeper than this. These realms are regions of mystery to most surface-dwelling creatures. Information about them is based on speculation or the few first-hand accounts of adventurers who have visited these underground realms and returned to tell the tale. It is known that certain races, such as the drow elves, deep gnomes, duergar, kuo-toa, and derro, to name a few, live in vast underground reaches that never see the light of day. The true extent of such realms can only be guessed at. These realms contain vast networks of caverns that might rival an entire nation in size, and underground bodies of water as big as any outerworld sea. These seas contain islands just as surface seas do.
Certain of the vast realms could best be described as wilderness, for no cultures flourish there, while other areas boast cities and fortresses worthy of the most advanced civilizations. Many of the creatures inhabiting these realms have been encountered by surface dwellers in the meeting-ground of the dungeon, for this is where the deep-dwellers and surface dwellers are most likely to encounter each other. Other creatures horrific beasts that never even venture up to the level of the dungeons-are rumored to live in these underground realms, but, no one living can attest to their existence.