What does an underground cavern look like?

That is a much harder question for a caverto answer than you might imagine!

First though you do not need the adjective.Caverns are underground bydefinition!

Each cave is unique but they have common features that include any of passage profiles related to their development, erosion details, sediment deposits, boulder piles, speleothems (fancy Word that encompasses stalactites, stalagmites and related deposits), streams, pools, waterfalls(or no water at all), wildlife, geological details.....

One thing that seems obvious is the darkness. Once away from the daylight, if you turn off your lamps,the darkness in a cave is absolute. Even experienced cavers won't normally wait underground in the dark for long before putting at least a small lamp on. It plays a strange trick on you. Hold your hand in front of your face and you think you can see it as a sort of different blackness.

I suggest you ferret around the internet for photos of caves. You can find them on tourist publicity for show-caves but try to ignore the artificial paths, steps and lights, or on caving-club web-sites, etc.