Since the 2024 solar eclipse is nearly upon us, the Boss and I felt it was high time to visit family in Texas. While here, we also took the kids (and, let’s be honest, ourselves) to the conveniently located Inner Space Cavern in Georgetown, TX. The 1-hour “Adventure” walking tour was both fun and educational, but it also gave me a lot to think about for cave-themed adventures.

First of all, I had forgotten how wet caves can be. I don’t know if this is the case with every cave, e.g., lava tubes, but most of the natural cave complexes that I’ve visited were carved by water over millions of years, and the water is usually still there. Our tour ended at the edge of a gorgeous underwater pond that, for all I knew, continued on well into the darkness.

From an adventure design perspective, this suggests a couple of things. A wet cave is a humid cave. I can only imagine how uncomfortable it would get trudging through such an environment with a pack of supplies and armor. So adding Exhaustion or some similar effect to a cave expedition would make it more realistic. Would it also make it more fun, by putting the party at a disadvantage against seemingly weaker-but-well-adapted cave dwellers?

Also, and perhaps more importantly, the cave is slippery. Seemingly every surface is covered with water or (worse) slick mud, which makes moving quickly and confidently challenging. From a combat perspective, most of the cave would qualify as rough terrain, at a minimum. And for a fun environmental challenge, I could treat certain areas as if affected by the Grease spell!

Second, I had also forgotten how irregular and, for lack of a better term, loop-y a natural cave system can be. We were mostly walking on a paved surface, but on either side, there were, at frequent intervals, branching tunnels both above and below us, all of them connecting to each other at various points, many of them partially flooded. At one point, we passed a large slope of mud that, we were told, was the remains of an Ice Age sinkhole that had let animals into the cave a hundred thousand years ago, and then later sealed up again. We saw the original drill hole that had been carved into one of the cavern roofs, allowing the caves’ discoverers to enter. 

This makes me think about “Jaquaysing” dungeons by adding multiple entrances and multiple paths through a dungeon: Natural caves are a perfect embodiment of that design philosophy. The Inner Space cavern complex has multiple entrances that we know about. It also has multiple paths branching off of each entrance, each one leading to a unique region of the cave, with its own network of paths. Near our entrance, there was, for lack of a better term, a natural ledge overlooking our path with a cave behind it, that we could only see because it was illuminated from behind. (A natural secret door?) I found myself thinking, “That would be a great place for a sentry or ambush. You know, maybe a couple of goblins with short bows?”

So if you’ve never been in a cave, I encourage you to try to find one and visit it. It is fun, educational, and (maybe) changes how you think the cave encounters in The Lost Mine of Phandelver and other adventures should feel. And in the meantime, here are a few natural cavern-inspired scenario ideas that I’ve thought of since the tour:

• A surface town is blissfully ignorant of the (humanoid monster of the week) that lives deep in the nearby hidden cave complex, and vice versa. However, torrential rains (perhaps the result of a weather changing spell that the PCs cast to help the town out of a drought?) have flooded the depths of the cave, forcing the creatures to “retreat” to the higher areas of the cave and seek food by night on the surface world, putting them in conflict with the terrified townsfolk. Can the party figure out and resolve the cause of the conflict, or will it end in terrible bloodshed?
• A malevolent (or just aggressive) amphibious race makes its lair in a natural cave complex that is mostly underwater. Can the party navigate the mostly submerged, pitch-black tunnel system to put an end to the threat, or will they become a victim of the environment as much as the monsters? (See the adventure scenario The Final Enemy in the book The Ghosts of Saltmarsh for one model of what that could look like, although that adventure is in a manufactured environment, rather than a natural one.)
• A sinkhole millenia ago trapped some surface animals underground. The party has need of one such animal (as a magical reagent, or to fulfill a prophecy, or as a familiar, or whatever), but they are now extinct on the surface. A legend leads the party to the cave, but how have the animals evolved over the years to adapt to their new environment, and what potentially dangerous creatures live in symbiosis with them?
• An explorer lost his way exploring a cavern, and now haunts it both as a ghost (which can be communicated with) and a hungry corpse (which cannot). The humanoid inhabitants of the cave send an emissary to the surface world, asking for help removing the undead predator that now stalks their people. The party needs to find the (hopelessly lost) ghost and convince him to follow them back to his body to vanquish it for good.

What ideas for natural cavern adventures do you have? Share them in the comments!