I previously discussed how jointing in rock often does not follow a perpendicular plan, and that a mine or cavern following that jointing will reflect that.
Since the joint sets like the ones pictured above are not perpendicular, an alternative for fleshing out this environment is the use of isometric grid paper. (Isometric graph paper via http://www.printablepaper.net/.)
Below are a few steps for making up a small mine sub-level or mini-dungeon. Obviously this can be expanded to be as mini- or mega- as desired.
I sketch out a few random lines on the grid for the framework of our level:
Then sketch out some corridors and rooms using the original lines:
Finish out the corridors, add a bit of roughness, throw in a couple more chambers, and add other features (a few shafts in this case.) The loop and dead ends can make for challenges and danger, where the PCs may get flanked or cornered (or do the same to an opponent). Cover one or more of the shafts as a trap/objective hazard.
Connect to other levels, populate, bake at 375 degrees for 20 minutes, etc.
I am no Dyson or Matt Jackson, that's for sure...