In role-playing games the cave-in, the collapse of the ceiling, let it be in a cavern or a dungeon, in a temple or ruins, is a great risk to the party of players. Cave-ins are unique to the underground environment. Although not a common occurrence, the chance that the roof of an underground location may cave in is something that must be considered, especially if characters are excavating, using explosive magic, or otherwise placing stress on the overhanging rock.
Although the usual effect of a cave-in is to bring the roof of an area crashing down upon its inhabitants, cave-ins can also endanger characters in other ways. For example, a bridge that characters are crossing may collapse, or a floor may give way, dropping those who were standing on it amid tons of jagged, crushing rock. Characters may even try to trigger a cave-in as a means of attacking an enemy; such attempts must be made cautiously, however, since a cave-in may create a chain reaction that collapses a far larger area than intended.
Causes of Cave-ins
Most underground settings are solid enough not to collapse unless subject to a great deal of external pressure. In general, a strong cavern, dungeon, or realm is only susceptible to cave-ins as a result of natural disasters such as floods, earthquakes, or volcanic eruptions. Certain magical spells that duplicate these natural disasters can duplicate their destructive effects as well. A raise water spell that creates a flood in a tunnel, for example, should be treated as a natural flood. In the case of a major natural disaster, the DM’s or GM’s judgment must be used to determine the exact area of effect.
Although the guidelines in this section serve to give an idea of what might occur during a cave-in in a specified area, the far-reaching effects of a major earthquake might actually annihilate an entire subterranean realm. Occasionally underground areas are ripe for cave-ins. Perhaps aging has caused much of the supporting rock in an area to crack and crumble, leaving only a thin area of solid support-or maybe a recent earthquake or flood has left an area so weakened that only a slight additional bit of pressure is required for the entire area to fall inward. Such areas should be noted when the maps for an underground area are prepared.
Only the frailest of underground locations are susceptible to accidental cave-ins. These collapses are triggered by the weight of a creature walking across a weak spot, or even by the sounds created by characters or creatures. Based on the weakness of the area under consideration, the DM or GM should assign a percentage chance of a collapse occurring. A ledge, for example, might have eroded to the point where it has a 5% chance of collapsing per character crossing it. This chance should be modified if the characters are very heavily laden or accompanied by mules or other creatures. The chance may be doubled if the characters run across the ledge, since the pounding is much harder than if the party walks slowly.
If the characters participate in melee combat on such a fragile surface, not only must the weight of all participating creatures be figured in (figuring carefully the weight of nonhuman creatures such as ogres), but the chance of a collapse should be doubled because of the additional pounding that the surface takes. In cases where the weakened area is not directly beneath the characters, the chance of a cave-in is reduced. Unless the characters apply weight to the area, sound is the only other likely avenue for causing an accidental collapse.
An area that could be triggered into a collapse by sound waves is very fragile indeed! The intensity of the sound must be considered, with a percentage chance of collapse assigned according to the loudness and sharpness of the noise. For example, an area might stand no chance of collapsing under the sound pressure of normal conversation, but the clashing of swords in combat might give a 1% chance per round of a cave-in.
Cave-ins Caused by Excavation
Whenever characters or creatures are digging into the earth, they run the risk of excavating away a significant support for the ceiling, causing a cave-in. Mining is the most common cause of this type of cave-in, and precautions such as shoring up an excavated passage are usually taken by the miners. More details on these procedures are provided later in the rules for mining.
Characters trying to dig an escape tunnel or clear rubble from a previous cave-in are often unable to properly shore up their excavated passage. In this case, weakened surfaces stand a fair chance of collapsing. Generally, the possibility of a cave-in under such circumstances is about double that of an accidental cave-in. There is about a 10% chance per turn that a weakened ceiling or tunnel wall collapses during excavation.
Cave-ins Caused by Sabotage
It is not unusual for characters or underground denizens to intentionally weaken an area so that it caves in at an opportune time-at least, opportune in the minds of the saboteurs. As with all types of cave-ins, the success of an attempt to purposely weaken a structure or surface depends on the inherent strength of the area. Many areas subject to such sabotage collapse on the heads of the saboteurs themselves.
Columns that support a ceiling, bridge supports, or narrow layers of rock over which characters or creatures walk are obvious targets for sabotage. To chip stone away to the point where a collapse is likely requires some skill at stonework and a little luck. A character must have miner proficiency to have a decent chance of successfully sabotaging an underground area. A character without miner proficiency who makes a sabotage attempt should have a 1/3 chance of successfully completing his task, a 1/3 chance of leaving the structure too strong to collapse, and a 1/3 chance of bringing it down upon themselves. A character with miner proficiency has a 2/3 chance of successfully sabotaging the area, and a 1/6 chance of not damaging it enough, and another 1/6 chance of damaging it so extensively that it collapses during the sabotage attempt.
Of course, an area-or at least the stone that supports an area-must be reached in order to sabotage it. The soaring ceiling of a vast underground chamber is very difficult for a character to damage. The columns that support that ceiling are much more accessible. These attempts take considerable time and create quite a bit of noise. See the excavation rules (page 50) for an indication of how long it might take for a character to chisel his way through a column that is three feet in diameter. The DM or GM must determine how much work must be done to sabotage a structure.
Random Occurrences of Cave-ins
If a campaign focuses on the underground for extended periods of time, the chance of a naturally occurring cave-in affecting the characters increases. Even though the chance is not very great at any given moment, if characters spend enough time underground, they are certain to eventually encounter some sort of cave-in.
The base chance for a cave-in during the course of the characters’ underground exploration is 1% per day. On long expeditions, the DM or GM may wish to alter this roll to a 10% chance rolled every 10 days.
Cave-ins just don’t happen on top of characters and their party: it could happen behind, in front of, out of sight, but in hearing range, an entrance, an exit, etc.
Cave-in Chain Reactions
Many things can go wrong for the characters if there is a chain reaction. A significant menace is created when even a small section of an underground location caves in, even if the cave-in does no immediate harm. The force of gravity is constantly attempting to work its destruction on the ceilings of the underworld; even a small cave-in can so weaken supporting structures that a massive chain reaction of collapse begins immediately or threatens an area in the near future.