Cavern 20!

 Cavern 20

Two or three chambers connected by a single long shafts and off-shafts are what characterize many caverns, but this one has some interesting passageways to the left, and the singled out shafts like the one on the top left and the smaller on on the right make this slightly interesting. This could be a simple vertical abseil down, but investigating the cavern/mapping it out could be more complex, and this would make it a great middle cavern, or a cavern between larger chambers. It is 660 ft in vertical height.

The edit changes the surrounding stone, and fills the actual cavern with coal. A gigantic coal vein, and a tunneled burrow between the chalky, squeaky, sooty, and cloying coal-vein.

 What creature would tunnel/burrow through coal?

If the players go through they may find out, and in the process they will get covered in coal dust. Face masks may be important to not get a temporary cough. Also any flame is INCREDIBLY DANGEROUS. if this were to be set alight, everything above would slowly die of noxious fumes, and anyone nearby would be cooked sooner than later. This vein could also be a contested area, as coal might be very valuable, and gathering some could be a offense to local groups/ a boon for trading or survival.