Did I mention yet how the sketchbox dice tool can be used for mapping out underworlds? I didn't? Man, that was the whole reason I started updating it, too...
The basic idea is that you use the techniques for randomly rolling village layouts to generate a dungeon or cavern system. Roll dice with pips on the sheet, each pip represents anything from one room cluster to one neighborhood; connect districts with tunnels instead of roads. Pretty simple.
For small cave systems, roll 1 die and treat it as a hamlet (each pip represents one to six caves; roll a d6 for cave layouts or randomly select a cave geomorph for each pip.) Alternatively, you could roll for elevation with d4s and roll 3 dice for cave layouts at the same time, as if you were rolling for above-ground terrain features, to establish the levels; any "level" without any six-sided dice is one large cave, and tunnels slope up/down to connect dice on different levels.
More extensive underworlds are rolled like villages or towns, treating each pip as a block of buildings or a district of 1d6 blocks. This is based on the assumption that such underworlds are the underground remnants of previously-above-ground settlements. For now, we're just concerned with rolling dungeon layouts, but in a future post I'd like to delve into interpreting the rings, rolling to see the status of tunnels and districts, and even how to do this on-the fly, so that you could leave your megadungeon notes as sketchy as the impromptu towns.