Undercity of Mog

Campaign ideas evolve as you develop, at least they do from time to time. I was working up a megadungeon environment for Mog when I realized: duh, use the lost river kingdom idea.
So two , yup two campaign ideas in one.

The 20 question style overview is being reworked of course. Having a vast underground ancient metropolis underfoot will hopefully keep players involved and provide a dungeon big enough to come back to time and time again.

Now with something like an undercity there can be vast wild areas, populated neighborhoods, and strongholds. The occupants look look like they are going to be a mix of classic D&D monsters, mutant future freaks,  labyrinth lord lovecraftian supplement, and David Bohman's beastiary for the dismal depths/blighted beyond. A slightly wacky and eclectic mix that will hopefully work to make the campaign a little different from my last one.

Treasures are getting the unique magic item approach from Pars Fortuna along with alien relics and ancient high tech gadgets. Coins are going to be an odd mix of metals and coin types with a mix of coins from different lands and times.

So I'm going to have a swords and sorcery surface world atop a vast megalopolis buried ages past full of unspeakable horrors from beyond, degenerate fey, ancient warmachines and dwellers in the darkness.

Only thing is... how does one map the ruins of something the size of megacity one ?