As a family we hit some of the Blacks Hills caves and most of the tourist Arkansas caves plus Marvel Cave at Silver Dollar City (Branson).
The take-away was that fighting any kind of monster in a cave would be quite difficult unless you wre in one of the rare flat and high-ceilling rooms.
I have my poster maps of Mammoth Cave and Wind Cave (SD) for maze-like caves.
I've been to Mammoth Cave a number of times, since I live only a little over an hour away. There are definitely lots of places in there where fighting some monster would be practical, plenty of large rooms and such.
Every time I tour there, I am mentally taking notes of ways to improve my caves for future games. Some ideas:
1. Varying terrain. Cave rooms slope a lot, some areas are going to be high ground, some are going to be low ground, it's not all a level dungeon floor there. Also, some slopes have broken rocks on them, just moving around or standing your ground may require a modest Dexterity check/Balance Check/Reflex save (whatever is appropriate to your game/edition) to avoid falling down.
2. Go vertical. There are shafts leading to the surface with water seeping in, and deep shafts going into underground lakes. Bad places to fall, good places to get ambushed from above.
3. Dark. Really, seriously dark. This cannot be emphasized enough, down there it's darker than anything you'll ever see otherwise, even in a "dark" room on the surface or the dead of night on a new moon. It's so dark that, without artificial lighting, you will see random occasional specks of light as a biochemical trick of the eye. If the lights go out, for anybody who doesn't have darkvision, it's a level of darkness they've never seen before.
4. Crawl, climb and swim. It's a cave, not a dungeon. There can be places where it's a lot less than a 5' by 5' square to get through. There may be places where most races have to duck down, or places where you have to climb to get over obstacles, and a water obstacle is fair game.
5. Big. Cave systems can get huge, especially in fantasy. Mammoth Cave is over 400 miles of recorded caves. That's real-world, that's enough caves to keep a party busy for a campaign, especially when you add in the various inhabitants they would have in a D&D world. Don't be afraid to have a path wind on for a while, or throw in some amazing rock formation, or some little side room.
6. Re-use real-life. There are a lot of chambers in Mammoth Cave that had a significant real-world story to them, especially chambers relatively close to the surface that have been known before the mid 20th century. They generally could be encounters or adventure ideas for D&D.
a. One side chamber has graffiti on the walls written in charcoal, of letters and words. . .it is where slaves in the Antebellum age were being secretly and illegally taught to read and write. I'm sure an enterprising DM could come up with some equivalent to this illicit education.
b. There are several small stone cottages in the larger chambers that are what is left of an underground colony that existed for a while down there. It was founded to treat tuberculosis, and before antibiotics there was no cure, but they believed that the cool air helped, so a physician lived down in the cave with patients. Of course, vitamin D deficiency from living down there, plus the inevitable spread of the disease meant the plan didn't live in the long term, but for a fantasy race that could deal with the lack of sunlight a small hamlet in a cave system is definitely not impossible.
c. In the late 1800's a local minister had found a chamber that was particularly suitable (by his standards) for religious services. It was a room with a slight slope, and plenty of room for congregants, and it had a ledge overlooking the room where he could stand and address everyone, and a path out from that ledge away from the main room. This preacher conducted services down there regularly for some time. The D&D role of some religious group having secret worship services in a cave has lots of potential.
