World-Building #5

Caves, Caverns, and the World of Eternal Night

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This is about natural caves and caverns, not artificial catacombs and mines. Catacombs and mines have a human logic to their construction. Natural caves are random and illogical, thoroughly unpredictable and often inconvenient in their form.

(If you want to read a fantasy novel that uses real caves as an obstacle, try The Weirdstone of Brisingamen. I'd read it more often if that section wasn't so frightening.)

Most caves are formed by water cutting through stone. The few that are not are

  • jumble "caves," voids in piles of boulders. They are like a casual heap of blocks. Earth movements or landslides can collapse them. They have none of the pretty features of "real caves" and are usually above the real ground level. They are usually next to the slopes the boulders tumbled down. When they occur in moraines left by retreating glaciers, they often represent a spot where a large chunk of ice remained a while. When it finally melted, the rocks falling in on the space locked together. See, even these can be water-formed in a backward way.
  • volcanic caves, like lava tubes, result when lava hardens on the outside, but the still-liquid insides run out afterwards. Think of it like draining a cordial cherry and leaving only the chocolate coating. These can be surprisingly complex, while featuring lots of shattered volcanic glass (obsidian) and leather-shredding rock textures.
  • fissure caves, caused by rock splitting. The fissure may or may not reach the top of the rock. These are never extensive or complicated.

All the rest are eroded by water, like most geological features.

Ice caves form in glaciers by the same method: meltwater running through fissures, eroding them larger.

The most extensive and prettiest caves will be found in limestone: Kentucky, the Yucatan, Germany. In fact, 95% of all caves are in limestone. Cavers (who hate the name "spelunker") will delight in any hole in the ground, eager to crawl in and see where it goes. Pretty formations are nice, but many would far rather find the deepest, longest, most extensive, or newest cave. Many of the miles listed in a cavern system's extent is not tourist trail. It's crawling through nature's drainpipes. However, the general rule has always been that what counts as cave is something a human can get to, or through. Naturally, top cavers are skinny and often short.

Caverns can have individual rooms too large to light and photograph. The record to date is 162,700 square meters in the Sarawak Chamber of the cave Lubang Nasib Bagus, in Malaysia.

As of March 2013, the deepest cave in the world is Voronya cave in the Arabika massif in the Caucasus mountains in the Republic of Georgia, at 6824 feet/2080 meters deep. Lamprechtsofen in the Leoganger Steinberge massif, in the Northern Calcareous Alps, Austria, is #2, currently explored to 5354 feet/1632 meters.

The US has Lechuguilla Cave, 1632 feet/497 meters deep, and 101 miles/156.5km long. It was only discovered in 1986, and has a potential to go as deep as any. (You can see a map of it, color-coded for depths, here.)

Kazumura Cave runs a vertical 3602 feet with over 40 miles of explored passages, making it the most extensive lava cave in the world.

The largest, most extensive cave system is the Mammoth Cave/Flintridge system in Kentucky, 360 miles/5906 kilometers of explored passages, but only going down to 379 feet/115.5 meters.

Caves send offshoots in many directions. The deeper or longer the cave, the harder it is to mount an expedition to stay underground long enough to explore many. New passable cave may be found at any time.


Underground Jargon

Sound like you know what you're talking about by using the caver's vocabulary. This recaps the information that appeared in an article of mine published in Dragon magazine #249: isn't D&D all about underground places?

This antique engraving of part of the Gailenruth caverns demonstrates that most real caverns are not neat catacombs with ten-foot-square passages: they have holes and drops, high places and low, all quite random. If you followed the links to the maps of the deepest caves, you saw what looked like diagrams of a handful of threads left hanging in near-zero-G. The only constant is a tendency to run downward. However, since this isn't open ground, sometimes the only route for water is to fill a gap then be forced up a rising crack by the pressure of the water still trying to come down.

Some areas of some caverns are flooded, so that cave scuba has become a special part of caving. As long as a system still has water moving through it, if only a drop at a time, it is considered "live," still forming. Systems are "dead" when they have dried out. A dead cave is often easier to get through, lacking water hazards. In live caves, you may encounter rushing torrents, rapids, and towering waterfalls, besides placid dark pools and lakes of unknown depth or extent.

A cave is any gap. A large or complex cave is also known as a cave system. Open cave is any part that is exposed to the immediate outside world, whether or not a person can reach the opening or pass through it. A cavern, when not just a synonym for cave, is a large or highly decorated cave, the kind you can walk tourists through.

Limestone caves are the reason for karst regions. Karst topography is marked by lots of sinkholes, pits in the ground ranging from the size of a bathtub to one you could lose a house in (a sinkhole in Florida once did open up under a house: new housing developments watering their lawns to get them green can wash out the ceiling of an unsuspected cave). When a cave ceiling collapses, you get a sinkhole. This collapse is called breakdown, and may still leave the actual cave forty feet below the surface, or open it to the outside. The area collapses doesn't have to be paper thin.

Any passage that leads from the outside into a cave system can be called an adit.

Ceilings are the uppermost part of any opening, floors the bottom, walls the sides. A passage is an opening longer than it is tall or wide, especially when tall enough to walk in, a natural hallway. A smaller passage, requiring crawling, may be called a lead [leed]. A crawl is any low passage, even if it's thirty feet wide. A cave that opens out beyond the passages around it, a wide spot in the road, is a chamber. Generally narrow passages are called corridors when mostly straight and leading between chambers.

Siphons, wet or dry, are one of those places where water got forced upward and cut a passage. A normal siphon is a U-shape that dips down and goes up again, also called a duckunder. An inverted siphon goes up then down again, rather sharply. A wet siphon will look like a pool or hole filled with water: you won't know it goes anywhere until you get into it to explore. A wet inverted siphon will only be found underwater. It is possible for one end to be underwater in a pool and the top of it, above the water level of the pool, dry, and the farther side drained. A siphon and an inverted siphon right after each other makes for vertically S-shaped passages. How difficult these are to pass depends on if they are a close squeeze or you can walk upright along the horizontal part.

Dripstone, also known as flowstone, is the stuff of cavern fantasies. All the beautiful fantastical formations are made of this. Simply, water coming through limestone dissolves it. Each drop carries its microscopic burden of lime. Anywhere it dries out, it deposits this iota of stone. Over time, centuries, it can form remarkable shapes. Similar speleothems can be formed out of water carrying any mineral: you may have seen concrete dripstone or rustsicles.

Where dripstone forms from the floor upward, like candle wax built up in a stack, it's a stalagmite (stalaGmites Grow up from the Ground). Where it forms from the ceiling like icicles, it's a stalactite (stalaCtites Come down from the Ceiling). When stalactite and stalagmite meet, you get a column. A very skinny stalactite may be called a straw. Dripstone can form along a fracture line into sheets of rock, called drapery.

Labeled Speleothems by Dave Bunnell

A pool of water evaporates along its surface. This can result in fragile cave rafts being formed on its surface. If cave raft anchors to the sides, it forms shelfstone (if the pool dries out from under the shelf, it can make very fragile footing). Otherwise, when it grows too heavy, it will sink and become part of the bottom of the pool. Small pools that are frequently dripped into may develop little balls of limestone, called cave pearls. Baconstone is limestone that is white or yellow, streaked with red from iron oxide. When this forms drapery that can be lit from behind, it is particularly beautiful. There may also be cave flowers, calcite crystals that form in clusters.


Let There Be Light

Writers taking characters underground often create artificial light sources so it's the equivalent of going into a building with no windows but a lighting system. These have included glowing ceilings left by The Ancients or the ever-popular phosphorescent lichen or fungi.

Let's address the business they're always trying to avoid: carrying your own light sources and how long they last.

Bernard Mason provided us with the information everyone wants and can't find: what "a torch" is and how long it burns. He describes a stick about 1'/30cm long and 3/4"/18cm in diameter. The end is soaked and coated with pine pitch (brighter, hotter flame) or tallow (soft animal fat, giving a dimmer light).

Remember the 1-hour limit they stick you with in D&D? Mason, who used torches in the woods in the days before flashlights, says one such torch "burns all night." Let's call that a 6-hour minimum, 12-hour maximum, 9-hour average (it could easily be the longer limit: someone want to make some and time the burn?). So as long as you can keep a torch lit, you won't have to carry stacks of them with you for a trip of a day or two. Each weighs about a pound (less than half a kilo).

An ordinary Alpine candle-lantern, with glass cylinder; 2" diameter; telescoped, 4" high, open 6 1/2" high, brass with bail hanger up 10" high, weighs half a pound, less than a quarter kilo, without candle. It uses 1.5 ounce candles (.04 kilo), 3 1/8" long, 1 1/8" diameter. Each of them burns 9 hours. So this is probably close to a torch's end mass of fuel.

Surely some of you keep emergency candles in the house for power outages. The cheap ones at the supermarket are rated to burn 12 hours, and weigh no more than two ounces.

We also picked up stats on a traditional oil lantern, black enamel with shutters, silvered reflector, 8.5" high, 2.75" diameter, lens 3.75". Empty, it weighs a bit over a pound, exactly half a kilo. It holds about a half-pound of lantern oil. It will burn on one filling for 10-12 hours, and lights up an area 40-60 yards/meters around.

One entertaining thought is that your lighting may use up all the oxygen. In certain dead-end passages, this may be a problem, or if you're caught in a cave-in. However, natural caverns normally have natural airflows. In some odd passages, this can be quite a stiff breeze, but normally it is just a draft, circulating air. Natural caves don't get the firedamp of coalmines. For starters, they aren't in coal formations. But if you send your characters into old coalmines, be sure to check on their particular perils.


Some Basic Minerology for Planet Building


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STORMY WEATHER: Siting Jungles and Deserts


Siting Mountains, Rivers, Valleys, and All That

THE ETERNAL CITY: Do-It-Yourself Political Geography

Caves, Caverns, and the World of Eternal Night.

Some Basic Minerology for Planet Building