While I'm not a professional caver, I do love visiting tourist caves and reading about cave exploration. (My favorite non-fiction title in the genre is Blind Descent by James Tabor.) Here are 5 points about the nature of caves and caving that you can weave into your next subterranean adventure.
[Technical note: caves can be formed by lava, coastal wave action, and even acid-producing bacteria, but the vast majority are caused by slightly acidic water seeping into pre-existing cracks in a layer of limestone beneath the surface. It's these caves I'm describing in this post].
Caves are dark.
Obvious, right? But unless you've ever been in a cave and switched off your light source, you won't really appreciate the difference between, say, a darkened bedroom on a cloudy moonless night and absolute subterranean darkness, with nary a photon around to ping your rods and cones. Utter, unrelieved blackness works on the mind, it distorts perceptions of time and distance, and even when your light source is on, you can feel it crouching at the edge of your lamp's glow, like a wall of water intent on filling a void. When cavers switch off their lamps to conserve power during rest periods, it's not uncommon for their minds to start filling up the lack of sensory input with hallucinations, from dancing lights (yes) to fully-formed visions of faces and creatures looming in the dark. In a fantasy world, where you can't be sure if what you are seeing is a trick of the mind or a real menace, the darkness would threaten even more. A party of NPCs that rests in darkness to save on torches or light spells or lantern oil might just start freaking out after an hour or so, requiring Wisdom checks to stay calm.
Caves are wet and clammy.
Remember how I mentioned that most caves are formed by water action? In fantasy caves, water features are usually occasional nuisances or window dressing, but in a real cave, water is everywhere: flowing, gushing, seeping, trickling, and pooling, usually exactly where you want to be going. In nearly any steep ascent or descent in an active cave system, you're going to be climbing next to or even through a cascade of water. If you keep track of water rations in your game, I'd say that good sources of water in caves should be plentiful. That's one advantage.
It would be nice if the amount of water flowing through was at a constant rate, but remember, this water is largely runoff from rain. If the climate is at all seasonal, the cave will be a very different experience after a rain storm or during rainy season than in a drier time. Rapidly changing water levels make caves deadly.
It is nearly impossible to stay dry in a cave without specialized clothing. You and your gear are going to get soaked. And it's cold down there. Campers will recognize that cold + damp = hypothermia, a potentially fatal loss of core body temperature. Only the breeziest, most casual game systems can ignore the hostility of an environment like that. In Dungeonteller, you lose Luck (health/hp) every day you're in such an environment. A cave crawl could kill a party of PCs in a day or so even if they never meet so much as a goblin.
And because they're wet, caves are noisy.
Even a little water makes a lot of noise. Anything that wants to sneak up on you doesn't have to worry about being heard, and if you're a rogue, don't even bother to move silently when you're creeping up on a foe. If you want to talk to another party member, you're going to have to talk into their ear or shout at them above the ambient noise of the water. Not an ideal way of avoiding detection by cave creepies.
Caves don't respect the horizontal plane.
Levels? You must be joking. A real cave is a maze of angled crevices, holes, and caverns, and even in the larger voids, the floor won't be flat, unless seasonal floods are bringing in a nice soft carpet of sand or silt (just pray that you're not there in high water). On a larger scale, you do sometimes see a roughly horizontal alignment of chambers where water finds the way down blocked by a more resistant rock layer below. If you've ever seen my dungeon maps, you know that this is one of my big design points: that natural caves love to wander and connect at odd angles and tangents. Again, even in a casual game, climbing skill should come into play frequently in cave crawls.
Cave combat is hard.
In all but the largest voids, melee combat would be awkward. Room to swing is at a premium. If I were setting up a marching order, I would put one or two fighters in front with awl-pikes or short spears, and one or two in the back with their spears reversed in their hands to be ready to repel attack from the rear. I think the ideal cave weapon would be a spear that you could telescope or break down into several lengths. A couple spearmen could dominate a passage, with no worries about being flanked. Spell casters would become key in breaking such stalemates.