There are plenty of articles here, there, and everywhere on the care and feeding of a megadungeon. However, for those of you who would like something new, I propose the following: convert your megadungeon to a hex map! Why? Because Hex maps have an entirely NEW set of articles here, there, and everywhere on their care and feeding, so converting from one to the other puts you in a new mindset on running them and gives you a whole new set of tools to do so!
Like usual with my articles, I’d like to start this one off with a load of completely unnecessary and grueling math. However, I’m going to be merciful and gloss over most of it instead of throwing pages of formulas at you. I trust if you’re the kind of wonk like I am that wants it done with mathematical precision you can figure it out yourself, and if you could care less you’ll appreciate that I skipped to the good stuff. On to the hand-waving:
If you insist on doing this accurate to 4 decimal places, it’s easy enough. Model your megadungeon’s shape with a rough geometric analog. Find it’s volume. Divide that volume by the average height of a floor to get surface area (imagine you’re slicing it into 10’ thick slabs). Multiply by density of open areas (usually between .1 and .5) and you’ll have an estimate of the area of your megadungeon. Divide by the area of a hex, and you know exactly how many hexes are in your megadungeon.
For example, I’m working on a megadungeon set in a hollowed out volcano. It’s roughly a cone 1000m high with a base of approximately 3600m. That gives it a volume of 1080000000π m3. With an average floor height of π m (to keep the math easy), I have an area of 1080000000 m2. With 50% density (made by dwarves) that’s 540 sq kilometers, which is roughly 7 6 mile hexes.
From here, lay out your hexes. This will have to be mostly abstract as it’s unlikely that your megadungeon is a single huge plain or that it fits into hexes neatly. It doesn’t really matter too much if a “hex” is actually a series of floors, or several complexes that share a similar ecology. For example, in my volcano, there are no single floors large enough to be a hex all their own. It would take 3 of the largest ones to fill a hex. Even though this is the case, the way you lay out your hexes can help inform you of which layers connect to one another, the relative danger level of areas, etc…
Once you have your hexes mapped, now go out and find articles on hex maps and go from there. You could go old-school macro with each hex getting it’s own unique pervasive environment type (this is the hex with the mud pits everywhere) or have multiple sub sections of each hex. You can build loose web-maps or slavishly detail everything. There are plenty of articles all over, but I suggest you start with Martin’s Old School Fantasy Hexcrawl Resources article.
The beauty of this system isn’t the particular way you transform the megadungeon into a hex crawl or the approach you take to your hex crawl. Rather it’s the new lens through which you view an old concept.