Causes of Cave-Ins	Random Occurences of Cave-Ins	Cave-In Chain Reactions	Effects of Cave-Ins	Defensive Points Values of Underground Formations
-	-	Cave-Ins (Mining)	-	-
DSG	-	-	-	PH

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 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.

Accidental Causes

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 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 himself.

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 must determine
how much work must be done to sabotage a structure.

Random Occurrences of Cave-ins

* The base chance for a cave-in during the course of the characters' underground exploration is 1% per day.
* On long expeditions, the DM may wish to alter this roll to a 10% chance rolled every 10 days.
* If a random cave-in is indicated, use Table 17: Cave-in Location to determine where, in relation to the characters, the cave-in has occurred.

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 may wish to alter this roll to a 10% chance rolled
every 10 days.
If the roll is successful, this means that a cave-in has occurred
somewhere in the underground region near the PCs. This does
not mean that the cave-in occurs on top of the characters, or even
within their earshot. If a random cave-in is indicated, use Table
17: Cave-in Location to determine where, in relation to the characters,
the cave-in tias occurred.

Table 17: CAVE-IN LOCATION

D100 Roll	Location
01	On top of PCs
02-03	Immediately in front of PCs
04-05	Immediately behind PCs
06-10	In front of PCs, within hearing range
11-15	Behind PCs, within hearing range
16-55	Well behind PCs, out of hearing range
56-95	Well in front of PCs, out of hearing range
96-00	At entrance PCs used to get underground

All directions relate to the party’s direction of movement at the
time of the cave-in. In the categories indicating a cave-in some
distance away, the higher the roll, the farther away the cave-in. It
is quite possible for a group of characters to fail to discover a
cave-in that occurs during their adventures.
In areas of exceptionally decayed stone, or of rotten supporting

timbers, the DM may increase the chance of a randomly occurring
cave-in. Such considerations could double, or perhaps triple,
the chance of a cave-in.

Cave-in Chain Reactions

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.

When a cave-in occurs, the areas of nearby ceilings and walls
must be checked to see if they weaken or collapse as a result of
the original cave-in. The DM should roll d100 and compare the
result to Table 18: Cave-in Chain Reaction. By comparing the
resulting number for each section of adjacent material, the extent
of the cave-in can be determined.

Table 18: CAVE-IN CHAIN REACTION

Original Location of Cave-In	AREA Above  Cave-In	AREA Next  to Cave-In
Ceiling	48%	32%
Wall	32%	12%
Ledge/bridge	5%	12%
Support Column	80%	4%

A d100 result that is equal to or less than the number listed indicates
that the adjacent area has also collapsed. The area of the
chain reaction collapse is about the same as that of the original
cave-in (see the following Effects of Cave-ins section). If the die
roll does not indicate a collapse, but is within 10% of the chance
listed, the area is significantly weakened, and might collapse at
any moment.

Areas that collapse as a result of a chain reaction force checks
for chain reaction collapses in all adjacent areas. The percentage
chance for such a tertiary collapse is half (rounded up) the
chance for the original chain reaction cave-in, If the chain reaction
continues, each subsequent roll is made with 1/2 the chance
of the chain reaction roll that forced it.

If an area is exceptionally weak and decayed, however, the DM
may ignore the declining chance of a chain reaction cave-in and
use the original cave-in chance for all subsequent rolls. This is
common in areas that have been heavily eroded by water or other
natural forces, or in areas that were constructed long ago and left
to fall into a state of ruin.

Example of Chain Reaction Cave-ins

A 9th-level druid casts a transmute rock to mud spell upon the
stone ceiling of a vast underground cavern. The spell affects an
area of rock 100 feet long, 80 feet wide, and 10 feet thick, immediately
converting all of the stone in that area to mud, which then
falls to the ground. The DM then rolls d100 for the area immediately
above the mud ceiling and each of the four areas to the
sides of the spell’s effect. The roll for the area above the cave-in is
a 31, indicating that another layer of rock falls on top of the mud.
This triggers yet another roll for the rock on top of the stone that
just fell, at 1/2 the original probability. This next roll is a 42, which
is greater than the 24 needed to continue the chain reaction
upward, so the second layer above does not collapse.

The d100 rolls to check the sides of the cave-in for a chain reaction
are 12, 65, 97, and 33. Consequently, only one of the four
sides of the spell’s area continues to collapse. As it does so, however
however,
additional checks are required for the area immediately
above this secondary collapse, and for the areas to the three
sides that have not already collapsed. All of these rolls have only
1/2 the chance of collapsing listed on the cave-in Chain Reaction
table, since the cave-in triggering the checks was itself a chain
reaction.

If a collapse should be triggered by a secondary check, it
causes additional checks to be made as well. These tertiary
checks, however, have only 1/4 the chance listed.
 

Effects of Cave-ins

The effect of a cave-in upon characters and creatures that are
in the area depends on where they are and the amount of rock
that is falling.

If characters are on top of something that collapses, consider
the distance fallen. Also, treat the character’s landing surface as
rough, as explained under the expanded falling rules (page 19).
The base area of a cave-in can often be determined by the
structure or surface that is collapsing. A bridge that collapses will
collapse along its entire unsupported length. A tunnel ceiling collapse
can never be wider than the tunnel itself.

In areas where the size of the cave-in must be determined randomly,
such as the ceiling of a vast chamber, assume that the
base cave-in occurs only in an area of 20 feet by 20 feet, roughly
in a square shape. Chain reactions must be checked for each
adjacent square area, as well as in the ceiling directly above the
rock that has caved in. Such chain reactions radically increase
the total size of a cave-in.

If a layer of rock falls from a ceiling or wall, assume that the
layer is three feet thick unless other considerations apply. A twofoot-
thick layer of stone that serves as a ceiling in one chamber
and a floor in the one above it, for example, is never more than
two feet thick when it collapses. If a three-foot-thick section of collapsing
rock leaves additional stone above it, then that stone
should be checked for a chain reaction cave-in.

If characters are beneath an area of rock that collapses, the
average base damage per character is 4d8 per 10 feet (or less)
thickness of rock that falls. The characters should be allowed to
make a saving throw vs. petrification for half damage. Thus, if a
30-foot-thickness of rock came crashing down upon the characters
in a cavern, each character would sustain 12d8 points of
damage unless the saving throw reduces the the damage to 6d8.
If the falling material is mud, silt, or any other sludgy substance,
the damage inflicted on characters below it is only half that caused
by falling rock. Characters are also entitled to a saving throw vs. petrification,
which-if successful-will halve this damage again. Falling
water also inflicts 1/2 of the normal damage, with a save allowed
for 1/4 damage. In the case of water, however, the damage is inflicted
for the first 10-foot-thickness of falling material only. Even if the character
remains under a column of pouring water, he does not sustain
adaitional damage from the weight of the water. It is quite possible,
however, that the character is unable to breathe and needs to
escape the water or drown.

Defensive Point Values of Underground Formations

Just as constructed fortifications have Defensive Point Values,
<unofficial term: Mega Hit Points: HP.M> <could be made a little more clear> <as well, add a link Siege Engines> <as well, add a link to Siege Attack Values>

as explained in the DMG (page 110), many types of natural and
constructed formations are assigned Defensive Point Values.
Use these values to determine if an area is destroyed by the
effects of an attack or spell.

When a Defensive Point Value is provided for a cubic open
space, the value equals the amount of damage needed to cause
the area’s ceiling to collapse. Check for additional ceiling or wall
collapses with the normal chain reaction procedure.

Table 19: UNDERGROUND STRUCTURE
DEFENSIVE POINT VALUE
 

Formation or Structure	Defensive Point Value
Room, 10-foot cube	50
Room, 20-foot cube	90
Column, 3-ft diameter	10
Column, 10-ft diameter	100
Wall, 10-ft square	20
Bridge, small wooden	12
Bridge, small stone	40
Bridge, large stone	80
Rock ledge, per 1-ft wide	10
Ceiling, small cavern	100
Ceiling, large cavern	250
Stalactite, small	8
Stalactite, large	20

 

SUMMARY

CAUSES OF CAVE-INS \ ACCIDENTAL CAUSES
* A ledge, for example, might have eroded to the point where it has a 5% chance of collapsing per character crossing it.

* 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 cave-in.

CAUSES OF CAVE-INS \ CAVE-INS CAUSED BY EXCAVATION
* There is about a 10% chance per turn that a weakened ceiling or tunnel wall collapses during excavation.

CAUSES OF CAVE-INS \ CAVE-INS CAUSED BY SABOTAGE
* 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 himself.
* 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.

* See the excavation rules (page 50) for an indication of how long it might take a character to chisel his way through a column that is three feet in diameter.

RANDOM OCCURRENCES OF CAVE-INS
* The base chance for a cave-in during the course of the characters' underground exploration is 1% per day.
* On long expeditions, the DM may [wish] to alter this roll to a 10% chance rolled every 10 days.

* If a random cave-in is indicated, use Table 17: Cave-in Location to determine where, in relation to the characters, the cave-in has occurred.

Table 17: CAVE-IN LOCATION

D100 Roll	Location
01	On top of PCs
02-03	Immediately in front of PCs
04-05	Immediately behind PCs
06-10	In front of PCs, within hearing range
11-15	Behind PCs, within hearing range
16-55	Well behind PCs, out of hearing range
56-95	Well in front of PCs, out of hearing range
96-00	At entrance PCs used to get underground

CAVE-IN CHAIN REACTIONS
* The DM should roll d100 and compare the result to Table 18: Cave-In Chain Reaction.

Table 18: CAVE-IN CHAIN REACTION

Original Location of Cave-In	AREA Above  Cave-In	AREA Next  to Cave-In
Ceiling	48%	32%
Wall	32%	12%
Ledge/bridge	5%	12%
Support Column	80%	4%

* A d100 result that is equal to or less than the number listed inidcates that the adjacent AREA has also collapsed.
* If the die roll does not indicate a collapse, but is within 10% of the chance listed, the AREA is significantly weakened, and might collapse at any moment.

CAVE-IN CHAIN REACTIONS \ EXAMPLE OF CAVE-IN CHAIN REACTIONS
* An Initiate of the 7th Circle (D9) <note variant> casts a [transmute rock to mud] spell upon the ceiling of a vast underground cavern.
* The spell affects an AREA of rock 100 feet long, 80 feet wide, and 10 feet thick, immediately converting all of the stone in that AREA to mud, which then falls to the ground.
* The DM then rolls d100 for the AREA immediately above the mud celiing  and for each of the four areas to the sides of the spell's effect.
* The roll for the AREA is a 31, indicating another layer of rock falls on top of the mud.
* This triggers yet another roll for the rock on top of the stone that just fell, at 1/2 the original probability.
* This next roll is a 42, which is greater than <> the 24 needed to continue the chain reaction upward, so the second layer above does not collapse.

* The d100 rolls to check the sides of the cave-in for a chain reaction are 12, 65, 97, and 33.
* All of these rolls have only 1/2 the chance of collapsing listed on the cave-in Chain Reaction table, since the cave-in triggering the checks was itself a chain reaction.

* These tertiary checks, however, have only 1/4 the chance listed.

DEFENSIVE POINT VALUES OF UNDERGROUND FORMATIONS
Just as constructed fortifications have Defensive Point Values, as explained in the DMG (page 110), <e><l>
many types of natural and constructed formations are assigned Defensive Point Values.

Table 19: UNDERGROUND STRUCTURE DEFENSIVE POINT VALUES

Formation or Structure	Defensive Point Value
Room, 10-ft cube	50
Room, 20-ft cube	90
Column, 3-ft diameter	10
Column, 10-ft diameter	100
Wall, 10-ft square	20
Bridge, small wooden	12
Bridge, small stone	40
Bridge, large stone	80
Rock ledge, per 1-ft wide	10
Ceiling, small cavern	250
Stalactite, small	8
Stalactite, large	20