In games, caves are thought of as homes for bears, a place to get out of the weather, where the dwarves live and drink ale, or as an underground path through a mountain range. But with a little effort, caves and other karst features can be used for more types of adventures or at least a good laugh while people are tripping in the darkness of the earth.
Karst is a landscape feature that is formed from the dissolution of soluble rocks such as limestone, dolomite, and gypsum. Bedrock dissolves through natural processes of rain soaking into the earth or by man made water flow through broken sewer and water lines. Karst is characterized by caves, sinkholes, and underground water systems.
Caves range greatly in size from the chair you are sitting in to a couple hundred miles of interconnected passageways. The larger the cave, the greater the dangers from roof collapse, getting lost, flash floods, and sudden drops. Cave floors are covered with loose rock, mud, and stalagmites, so trips and falls are common. Tripping is usually embarrassing instead of deadly if you aren’t in a rush, but if you are running with a sword in hand you could hurt yourself pretty badly. Stalactites and suddenly short ceilings are surprisingly easy to run into if you are busy looking at the floor. If you haven’t spent time in a cave with no lights on at all, try it. Total darkness is much darker than what most people think. Your light source goes out, and suddenly you are wandering aimlessly in circles, constantly tripping and bumping into things if you are lucky. If you aren’t so lucky, you can walk right off an edge to your death.
Cave ecosystems are fragile and unique, but rarely house large creatures due to passageways that quickly change size and general lack of food. Natural caves won’t bypass an entire mountain range either. Some help from magic, hard work, exotic creatures, or if it’s good for the story can change that. Even then, passageways will wind around and have varying heights, pools of water, and larger ones have rivers.
Sinkholes are another type of karst that some people are familiar with and difficult to spot where one will occur in advance. Sinkholes can be a hole or vertical shaft dropping tens to hundreds of feet to the bottom or as a shallow depression on the surface. Most of the time, they are caused by roof collapse or sagging of weakened partially dissolved bedrock. Some examples include sinkholes that suddenly open up and swallow a car, half a house, or a tractor in a farm field. Most sinkholes will hold and transmit rain underground. This may leave small ponds for periods of time after a rain. Sinkholes are a direct pathway for pollution to reach groundwater supplies, springs, and streams.
Caves and sinkholes aren’t the only entertainment karst can bring. There are times when a stream or river will suddenly disappear or shrink in size, known as a losing stream. This happens when a crack or sinkhole gets big enough to divert the river underground through a cave system. Though the holes are almost always too small to lose a ship in, there are some losing streams in jungles around the world that could eat a ship. At others you can hear the waterfall dropping into a hidden cave system. If a character has a lot of knowledge of the area, they can let their stolen goods conveniently drop into one of these locations and pick it up from the cave system after the guards have stopped chasing them.
When water goes into the ground, it has to come back out. Springs and streams that start from nowhere or suddenly grow from small streams to full fledged rivers are also common in karst areas. Just so you can have an idea of the magnitude of some springs, Big Spring in Missouri is one of the largest in the United States and discharges 3,500 gallons per second on average. That water is dissolving approximately 175 tons of limestone bedrock every day throughout its area of influence. Springs are a great location to start a town, since it is usually clean and can be a reliable water source. Droughts and pollution upstream can ruin a springs usefulness as a source of water in a very short time frame though.
Without knowledge of the rock types in an area or geophysical instrumentation, characters are going to have some difficulty determining if karst is common in the area. They may see a cave or a sinkhole and assume that more are in the area. Another indicator is a lack of surface streams. If most of the water is running underground, there isn’t much left to travel along the surface.
The most common physical damage karst can do to a character is falling. Some falls will be just a foot or two while others could be 50 feet or more if you are in a larger cave system. Falling is described in more detail in Dr. Gentlemen’s article The Realities of Falling: Giving Gravity the Respect It Deserves. Drowning in flash floods and starvation from getting lost underground are also deadly. Karst can indirectly do damage by breaking utility lines (by removing the rock and soil supporting them), structural damage by settling, or by transporting pollution quickly.
If your game is a magic heavy game, and you have rock golems or dwarves who live in the mountains, they can expand the natural cave systems into great underground cities. In the process they have to deal with water seeping into the caves, supporting the ceilings and walls, and figure out a way to feed themselves underground. Games of ritual and superstition can find great significance in large sinkholes (like the Cenotes in Mexico) or caves with ancient writings. If a sinkhole occurs and turns a great and mighty river into a small stream, there are big social and economic issues to consider, as well as who angered the spirits.
Though of karst is generally thought of as some of the less glamorous natural hazards, I hope this article has given you some ideas to surprise your players.