Friday Encounter: The Limestone Caves

As far as geological phenomena go, I've long held the belief that limestone caves and similar formations are woefully underused in fiction in general, and RPGs in particular. Limestone is known for collapsing without warning as it's eroded by water, which can lead to the structure of limestone caves changing dramatically in short periods of time. This could make for a dungeon that can't easily be mapped, and which presents dangers when navigating - one could find themselves sealed in a cavern if the entrance collapses behind them, or they could even be caught in the collapse of a tunnel while it happens. To say nothing of the risk of the caves filling with water! All in all, being trapped in a limestone cave is horrifying, and something that deserves to be utilized more.

This encounter aims to mechanically represent the dangers of an unstable limestone cave system. It can be used in a variety of different ways. The cave entrance could simply be dropped in the wilderness, along a road, or even in a town, or the ground could suddenly collapse as the PCs pass by to reveal a cave entrance! You could even use limestone caves as an extension of an existing dungeon. If the PCs aren't the types to investigate a cave out of curiosity, you could have NPCs impart rumors of treasure being hidden in the caves, or perhaps a shepherd hires the party to track down a sheep that went missing there.

I've provided a map of a sample limestone cave, but you're welcome to make your own. You could even spin the concept out into an entire dungeon, stocked with traps, enemies, and puzzles accordingly. There are six rooms, marked with numbers on the map, where an objective might be located - this could be treasure, or a missing sheep, or whatever the PCs might want to venture into the caves for. There are also 13 passages linking the rooms.

Click here to enlarge

You'll need to do some rolling in advance here. First, roll 1d6 to determine which room the objective is in. Then, roll 1d4 for each of the passages. The result of the roll is the number of times that passage can be passed (by the entire party) before it collapses. Once the count is up, roll 1d6. On a roll of 1, the passage collapses while the PCs are in it, and everyone crossing must make a DC 10 Dexterity saving throw or take 5d10 bludgeoning damage from the falling rocks. On a roll of 2-6, the passage does not collapse until after the entire party exits through it.

A collapsed passage cannot be crossed. This, in effect, makes navigating the caves a tense experience - you can never trust each passage, and the layout is rapidly changing around you, so the route you took to get in might not work to get back. It is entirely possible for the PCs to get trapped at a dead end. This might lead to a TPK if they don't have access to teleportation magic or other means of getting out, so a more merciful DM might want to add some other exit - perhaps a hidden trap door that leads to a larger dungeon below. Then the PCs will have to brave its challenges to find the way out.

Another option is to have the rooms fill with water once the PCs enter. I recommend using this for the room with the objective, for maximum tension, but you could also use this mechanic as an alternative to having the tunnels collapse - have them start filling with water once the number of safe passes is up. Once a room or passage starts to fill with water, enter Action Time. After each turn (not each round!), the water rises by 2 feet until all PCs leave the room or passage. Hopefully, the PCs can get out quick, or else hold their breath...