Six Cave Perils

Monsters aren’t the only thing adventurers need to be concerned about when they crawl down into the pitch-black world of natural caverns. These geological wonders can be a hazard and a hindrance even without their monstrous occupants.

1. CEILING COLLAPSE
It takes an engineer to make a stone ceiling that doesn’t collapse. Rock is heavy after all, and the pull of gravity and even erosion can bring that rock crashing down. Likewise, when a spell goes off, or a thunderstone detonates or even a loud noise rattles through a cavern, its ceiling often feels the stress.
The thunderous sounds created by spells can trigger a collapse. Most spells, unless specifically muted, create a sound or vibrational effect, and spells that have an explosive effect create even stronger shockwaves.
These activities can cause the ceiling, or at least part of it, to collapse. Such an avalanche of stone can be perilous for adventurers, who are often more preoccupied with hunting monsters than the structural integrity of a geologic formation.

To determine the potential disruption level of a spell, follow this formula:
10 + spell level + miscellaneous modifiers = Disruption level.

Miscellaneous disruption level modifiers

• + 10 explosive effect
• + 10 if the spell has a sonic descriptor
• +5 if the spell has an earth, fire or force descriptor
• + 5 per 10 foot of radius the spell effects

Other activities create their own sound which can trigger collapses. These modifiers are cumulative with spell sound levels.

Disruption levels for other occurrences

• Movement: +1 per 20 medium-sized creatures moving under the same ceiling.
• Battle: +1 per medium-sized creature engaged in battle.
• Work activities: +1 per 15 medium-sized creatures working under the same ceiling.
• Adjacent collapse: +1 per 10 feet of ceiling bordering collapse.

When the disruption level equals or exceeds the break DC of a weakened stone ceiling, it collapses, raining debris on all those below it. Damage is determined by the height of the ceiling. In such collapses, it’s rare that the entire ceiling collapses, but rather sections collapse in relatively thin layers.
Weak areas can be spotted ahead of time. Dwarves and other creatures with Stonecunning get that bonus to search checks when they’re within sight of a weakened ceiling
Ceiling Collapse perils can be formatted into a trap-like statistic block.

2. FLOOR RUBBLE
When a ceiling does collapse, the debris, called breakdown (shown in photo from http://www.goodearthgraphics.com), is rarely removed. This debris creates a walking surface that’s difficult to traverse and prone to shifting under one’s feet.
Such a landscape is considered difficult terrain, meaning that those traversing the breakdown have their movement halved.
Whenever a creature moves more than 10 feet along a pile of breakdown, it can trigger a breakdown shift and knock prone all those who are in caught in the mini-landslide.

Some creatures have learned to move across shifting landscapes, and have earned a feat to help them travel across it.

3. LOW CEILING
Caverns aren’t carved with medium-sized creatures in mind. While most creatures with humanoid physiques seek out expansive caverns to live in, other, non-humanoids aren’t so picky. As such, many caverns are left in their natural state, and these caverns might have ceilings so low that it’s difficult for a medium-sized creature to function.
At heights of five feet or less, medium-sized creatures are at a severe disadvantage when attempting attack actions, spell casting and some skill use.

• Five-foot ceilings — Medium-sized creatures must stoop to walk.
• Four-foot ceilings — Medium-sized creatures must bend over and have at least one-hand free to walk.
• Three-foot ceilings — Medium-sized creatures must crawl on hands and knees. Small-sized creatures must stoop to walk.
• Two-foot ceilings — Medium-sized creatures must slide on their stomachs or backs to maneuver. Small-sized creatures must bend over and have one-hand free to walk.
• One-foot ceilings — Medium-sized creatures must travel without armor and make Escape Artist checks to maneuver. Small-sized creatures must crawl on hands and knees.

Penalties

• Stoop: -5 feet on movement; -1 to hit; -1 to AC; Concentration check to cast spells; Strength-based skill checks at -2; Dexterity-based skill checks normal.
• Bend over: -10 to movement; -2 to hit; -2 to AC; -2 Concentration check to cast spells; Strength-based skill checks at -4; Dexterity-based skill checks at -2.
• Crawl: -15 feet on movement; -4 to hit; -4 to AC; -4 Concentration check to cast spells; Strength-based skill checks at -6; Dexterity-based skill checks at -4.
• Slide: -20 feet on movement; -6 to hit; -6 to AC; -6 Concentration check to cast spells; Strength-based skill checks at -8; Dexterity-based skill checks at -6.
• Escape Artist: DC 20 per 5 foot movement; attack actions impossible; -8 to AC; spell casting impossible; Strength-based skill checks at -12; Dexterity-based skill checks at -10.

Note: See Player’s Handbook entries for Squeezing p. 148 and Escape Artist p. 73. Those DC levels are for squeezing through a circular hole, not a “limbo-style” obstacle where you have plenty of horizontal space.

4. TRAPPED GAS
Low ceilings also have a tendency to trap noxious or explosive gases. These gas-trapping areas can be presented as trap-like encounters, and an open flame within 5 feet of the area sets off an explosion.

5. TIGHT SQUEEZES

Most natural caverns are carved by the seasonal flow of water. As such, water only needs a tiny space to squeeze through. Because of this, caves often have tunnels that are no more than a foot or two across. While some creatures can slide through such a space easily, adventurers wearing armor and carrying backpacks just can’t do it.
The squeezing rules in the Player’s Handbook (PH p. 148) shows how large or bigger creatures can shimmy through tight spaces, and these same rules can be applied to medium- and small-sized creatures. In general, medium creatures can squeeze through an opening two-feet wide and five-feet tall. Small creatures can fit into spaces 10-inches wide and 2-1/2 feet tall. To squeeze into such an area, a creature halves its movement rate and takes -4 to AC and attack rolls, and can move through a space about one-half the creatures’ face. Additionally, a small- or medium-sized creature can only perform “squeezed movement” into if she removes armor and other bulky items. Armor and items can be pulled through the space if tied to a rope.
By making an Escape Artist check (DC 30), a creature can move at its normal rate, but still faces AC and attack roll penalties. After a few such encounters in caves with tight squeezes, adventurers will soon learn the value of a grease spell, which provides a +10 bonus to Escape Artist checks in such instances.

6. VERTICAL SHAFTS
Though many DMs lay their caverns out on a horizontal plane, most caves have a slope that goes down. Again, this is because most natural caves usually drain water to the lowest possible point, usually an underground river or lake. Some caves drain vertically, rather than on a slope, and this type of drainage creates perilous shafts that adventurers must overcome in the midst of a quest.
Scaling such shafts can be difficult, as adventurers rarely pack enough equipment and supplies to travel down holes that can be hundreds of feet deep. Since rope is sold in 50 foot increments, most adventuring parties only have a few hundred feet of rope, which isn’t enough to go down tall vertical surfaces. Typically, whenever rope is used in climbing, at least one-fifth of its length is used in knots and harnesses. Another one-fifth is “used up” as it skirts around the bulging contours of a rock face.
Aside from ensuring they have enough rope for such a descent, water-slicked ledges, patches of fungus and animal nesting sites can pose trap-like hazards for spelunkers.