For most of the history of dungeons in Fantasy RPG a dungeon has been mapped out foot by foot, the specific location of every room and it’s exact dimensions has been noteworthy. there have been exceptions to this and the most famous being the D series of AD&D modules but in that even the vast domains mapped out under the earth were really little more than and adventure flowchart as beyond focal points the only difference was how wide and well used a route was. To have a dungeon that is truly a megadungeon it may be wise to move up in scales to have a macro dungeon where the general characteristics and likely features are of an underground area are described in as general a fashion as outdoor terrain is described at larger scale.
Here's some possible types of MacroDungeon terrain:
Passage/Tunnel- as with venerable source material an area defined as a Passage or Tunnel is much the same as noting where a road is on a surface map. The ease of travel may be indicated as can the frequency of use by locals.
Damp Caves- an area of damp caves is one where some water is still active and part of the cave formation, passages follow the flow of the water that slowly seeps through. While damp caves may have even present moisture they may never have adequate running water for surface dwellers but will be adequate for a variety of local life.
Dry Caves- an area of dry caves is a rocky desert far under the surface of the world. Areas will have tumbled and collapsed long ago or simply be waiting for hapless travelers to bump their heads or set a foot down wrong and cause and extensive collapse. Dry caves will have few if any local life and be a very dangerous place to travel to those unfamiliar with the terrain.
Mine- a mine is an excavation intended to aid in the extraction of ores and minerals from the earth. The specific nature of a area of mines will surely vary but the general function and traffic flow of an area of mines will be to get to good deposits and get those despots off to refining and to market. An area of Mines isn’t about crossing distance but locating the riches sought for. An area of mine will be defined by the materials begin sought, who was doing the mining, and how long ago the mining was begin done. An area of goblin or dwarven mines will surely be different to cross based on an adventuring parties relationship with goblins or dwarves but it may also be influence by the habits of the miners.
Maze- a maze in a Macro Dungeon is a twisting, turning mess of tunnels either meant to conceal something within, trap the traveller, or offer chance at enlightenment by the experience of traversing the maze. A maze is easy to get into but probably not easy to get out of. A maze area in a Macro Dungeon need not note every twist or turn in fine detail but is should always be made clear to those traveling in one of they are enroute as expected or stumbling about as minotaur bait.
Waterway/Undersea- seemingly not much different from a passage at first but underground waterways may be channeled into pipes or flow into natural channels where there are no air spaces or clearance to permit a boat to be used. All the hazards of exploring waterways on the surface must be considered but one also has to be contend with ceiling height. Another hazard with sunken waterways and underseas is that of chemistry, on the surface the chemistry of waters and where they flow is seldom an issue but deep underground the chemical reactions of water flowing through minerals can more readily cause toxic air, poisoned waters or even flows of acid.
Under Canal- as with canals on the surface and under-canal is expressly constructed for the purpose of moving goods quickly and more readily than would be expected on foot. A well maintained canal is far safer than a would waterway unless travelers are not on good terms with the canal keepers. Ancient canals may be used long after their original builders are gone and may hide threats from direct observation.
Mushroom forest- the fungal forest is a classical feature of underground exploration in fantastic realms and will offer similar challenges to traveling forest int he surface realms but the nature of riotous fungal growth will create different challenges as the fungus surrounding intruders try to feed of the energy provided by travelers, spores, molds and slimes may well seem to descend on travelers with a seeming carnivorous voracity even if there are no actual mushroom monster present. footing may be difficult, paths may quickly become overgrown in a riot of fungal growth, and the air itself can be full of opportunistic and deadly spores.
Fungal Groves- a fungal grove is a tame cousin of the Mushroom forest but as the mushroom forest is wild and untamed the fungal grove has been recently or is begin currently groomed for use by intelligent species. Travel through a fungal grove will be much easier than within a mushroom forest but the chance to encounter the locals is very high.
Slime Pits- A cousin of the Mushroom Forest but instead of more useful mushroom like fungus growth the area is covered in an array of slimes, jellies and oozes some of which may present little more hazard than slippery footing but some may be pools that can be drowned in or cousins to more familiar voracious dungeon scavengers.
Flesh Pits- a flesh pit is a vast area occupied by a massive organism or organisms. Passages and rooms are between natural openings in the body of the organism or inside the tissues and organs of the creature(s). A Fleshpit may appear to be more traditional caves and tunnels with an overgrowth of fleshy tissues or it may be like trying to sail through the belly of a leviathan or to squirm through it’s intestines.
Moss Caves- not as seemingly dangerous or impressive as a Mushroom forest or slime pits but they can present unexpected hazards because o fthe manner mosses can grow and support delicate structures. Moss can grow cover the surface of relative still water presenting a surface no different from that around it that can send one tumbling down into the waters to drown. Moss also attracts species of underground herbivores and the predators that feed upon them.
Root Forest- the roots of mysterious deep dwelling flora can reach far into the earth and erosion and other factors can exposes a vast stretch of roots or the roots can overgrow and choke off other spaces beneath the earth. Not all roots are as passive as surface dwellers may think as some burrow deep looking to feed on the flesh of sleeping dragons, slumbering giants, and great unnamed things
Crystal Jumbles- some crystal caves may seem little different from other caves but the environment and minerals that allow vast and extensive crystal growth can be very dangerous. Temperature, moisture, and occult energies can make crystal jumbles unpleasant or lethal. Those attempting to cross a Crystal Jumble can find themselves clambering on vast, slippery delicate structures high above pits covered in diamond hard shards or simply on floors covered in razor sharp slivers of broken crystals.
Lava Tubes- Lava tunnels are formed by the flow of magma and are not designed by any intelligence so are dangerous, convoluted and dimly difficult to travel without even considering the danger of a fresh flow of lava bursting out to flood a tube and incinerate anything within.
Lava Pits- an area of lava pits is hideously dangerous, the floor one walks on can be a mere eggshell above a lake of molten rock and fire. Molten lava can pour down from fissure far above and lakes of fire shoot out searing heat and toxic gases making travel to mortals madness. It is the very dangers presented by Lava Pits that draw some to them for the defense provided, the warmth, or the exotic nature of the molten minerals.
Vaults- these are not treasure vaults but vast chambers and halls expanded or excavated by intelligent folk. Traveling through vaults is not unlike walking down a major road or through a city park. If vaults are not abandoned and forgotten they will be a very busy place with much in the way of locals.
Gateway/Portal/Door/Bridge - such a chokepoint underground as a gateway, vast portcullis, drawbridge or well built door offers an obvious impediment to travel. those worth noting on a MacroDungeon Map will be significant affairs in scale or in utility (based on the scale of areas separated). For such a place to remain it must be useful and protected by some significant defense.
Catacombs- these are vast areas constructed to house the dead of a civilized people. They may be as confusing to traverse as maze and fiercely defend by locals once intruders are discovered rooting about in the remains of the dead. Catacombs may be trapped or conceal areas of entombed kings and queens remain surrounded by unimaginable wealth and deadly defenses.
Undercity- an undercity serves the same purpose as a city on the surface some may even be cities of old swallowed by the earth or transported by magic. A great danger of an undercity is it’s inhabitants and possible bizarre customs in an eat or be eaten environment. An under city will be shaped to house it’s inhabitants and engineered to provide for their needs along with the industry that provides (provided) purpose to the undercity. Factions may battle it out over control of the undercity or authorities may have walled off vast regions to retain control so even if an under city seems to be a populated civilized space it can be surrounded by dangers.
Bastion- a bastion is a fortress in the deeps. The corridors and halls in such a place are built to provide for defense of those within with communication routes, travel routes, barracks and armories for the garrison.
Rock- ever presents the very strata of the deeps provides a wealth of possibly fortunes and hidden miseries. Rock may be strong or fragile, rich in resources or of little material value.
Minerals- it’s worth noting the placement of vast mineral reserves concealed in the deeps under the earth. Such minerals are part of why the races of the deep remain hidden from the sun.
Bones/Remains- the locations where one can find the remains of dead dragons, giants, godlings, and forgotten begins of the past provide a wealth of opportunities both mundane and magical. What was buried with the dead demigod? What was in the crop of the ancient earth dragon? What wonders can be carved from the tusks of long buried giant beasts? More recent remains present a host of hazards and opportunities as well: Can a man breath in a dragon graveyard deep in the earth? the only way across a an undersea may be upon the floating carcass of a dead titan, beware the scavengers that slowly devour it’s rotting flesh.
This isn't all virgin territory in figuring out how to present really big underground realms and megadungeons in the deep depths of fantasy worlds but it's worth considering as a different way to present adventure location in a fantasy campaign. Depending on scale of time and distance MacroDugneon terrain is designed for a DM would have to consider the chance to get lost, travel times, the chance to find local resources, odds of random encounters/events, and the placement of lairs. (There is room for one or more other posts on all this).
a couple other megadungeonn posts:
Vast spaces in megadugneons: http://aeonsnaugauries.blogspot.com/2015/02/megadungeons-need-vast-spaces.html
Megadungeons need Neutrals: http://aeonsnaugauries.blogspot.com/2010/11/megadungeons-need-neutrals.html