How to legally trap a character under a rock, underwater

Question

The huge water elemental grapples the PC, and pulls him into the murky water. The rest of the party manages to finish it off with ranged attacks and spells, but oh no! It was a load-bearing elemental, and when it dies the cavern starts to collapse! A great big rock falls from the ceiling and hits the now-submerged player, pinning him to the pool floor. The party dives into the Devilfish-infested waters in a bold attempt at a daring rescue.

Let me start by saying I'm all for hand-waving for the sake of some good action and an intense encounter, especially if it throws the PCs into an environment to which they are not accustomed.

With that out of the way, is this situation possible to construct in Pathfinder in a completely legitimate way? Certain things certainly don't have to be explained by the rules, such as a cave-in. It's entirely possible that just happens naturally, without having to be explained by magic or a trap (but bonus points if it can be worked in somehow!)

1. Assuming the cavern can be made to collapse, what happens to the PC? Does the falling rock make an attack against the PC? A CMB check? Does the PC reflex save? Swim check?
2. Assuming the rock can be made to "hit" the player, does he just take damage, or is there a way to have it apply the grappled, or even pinned condition to the player at the bottom of the water? Is the player helpless?
3. If so, what means can the player use to escape? Strength check? Escape artist? CMB? What is a rock's CMD?
4. Can the other party members assist in a meaningful away? Assuming they don't have any magic that "solves" the encounter like turn rock to mud etc, what can they do if they've dived down to the bottom to try to help him escape? A measly +2 from "assisting" on some check doesn't seem that great, when realistically 3 people all shoving a rock should be 3 times as effective, if not moreso, than one person (who is trapped beneath it).

I appreciate that the answer to this question is "probably not," but I'd still like to see some attempts at legitimizing this action sequence.

Answer 1

You're in luck, 'cause there are actually rules in Pathfinder for cave-ins and collapses:

Characters in the bury zone of a cave-in take 8d6 points of damage, or half that amount if they make a DC 15 Reflex save. They are subsequently buried. Characters in the slide zone take 3d6 points of damage, or no damage at all if they make a DC 15 Reflex save. Characters in the slide zone who fail their saves are buried.

Characters take 1d6 points of nonlethal damage per minute while buried. If such a character falls unconscious, he must make a DC 15 Constitution check each minute. If it fails, he takes 1d6 points of lethal damage each minute until freed or dead.

The given CR for this sort of environmental hazard is 8, but I'm sure you'll be able to scale it up or down, depending on the party level and your intended deadliness of the encounter :-)

Edit:

Going deeper, as required by Eric B :)

On escaping the collapse, let me keep quoting the paragraph I linked to:

Characters who aren't buried can dig out their friends. In 1 minute, using only her hands, a character can clear rocks and debris equal to five times her heavy load limit. The amount of loose stone that fills a 5-foot-by-5-foot area weighs 1 ton (2,000 pounds). Armed with an appropriate tool, such as a pick, crowbar, or shovel, a digger can clear loose stone twice as quickly as by hand. A buried character can attempt to free himself with a DC 25 Strength check.

But how does the underwater situation come in to play? Apart from drowning, we have:

Digging people out: Because water is denser than air, objects submerged in water will have higher buoyancy than when in air. Thus moving rubble and rocks should be easier under water. However, unless our heroes are affected by freedom of movement or similar enhancements, movement is down to one quarter (assuming successful swim checks). Right off the bat, I'd say moving loose rocks underwater can be done at roughly the same speed as above water. Few large rocks would decrease the time, and a huge pile of rubble would probably take quite a bit longer to move under water than above.

Freeing yourself: Because stuff is easier to mover under water, I'd give the submerged character a +5 bonus on the strength check to free himself.

Damage from the collapse: Since damage dealt underwater is halved, I'd apply the same 50% discount to damage from falling rocks.

Answer 2

I'm not entirely familiar with Pathfinder, but it seems like the Earthquake spell combined with a spell trigger might be your best bet. Maybe the elemental was placed there by a druid, along with the trigger (If you can make a non verbal trigger, or make it a trigger word that is likely to be spoken by the party).

Effect of earthquake

Cave, Cavern, or Tunnel: The roof collapses, dealing 8d6 points of damage to any creature caught under the cave-in (Reflex DC 15 half) and pinning that creature beneath the rubble (see below). An earthquake cast on the roof of a very large cavern could also endanger those outside the actual area but below the falling debris and rubble.

As far as the escape, they have the saving throw, and/or if the pool was noticed to have ledges, the quicker/more astute can dive into the water and hide under the ledges, thereby precipitating the devilfish encounter. Or, you could even have the ensuing cave-in and rubble force them into the water through creative narrative.