Greetings, brethern.
I am actually working on simple but playable rules for a swamp holes ("sloughs")..or more correctly, for pc who end up in one. I am a little unsure about he whole affair, so input is very welcome.

Slough
Normally pc can avoid stepping into a slough by the use of the Survival skill. However, if the pc leading the group fails the skill one of the pc will likely end up in one. Even if the tests are passed, a GM can simple rule that a travel into a swamp is not complete without this sinking feeling. The fourth part of Indiana Jones introduced the “gas bubble beneath the ground brakes and suddenly you sink a lot quicker” syndrome. Which is quiet handy for GM purposes.

A pc whom stepped into a Slough will need to pass an extended test for Survival (or Dexterity/Intelligence, GM´s choice). The difficulty and number of success need to get out depends on how much of a hazard the slough is. Since the more dangerous might be the more easier to detect (my personal assumption), I suggest the following table.

Level of Failure (on the Survival Test) Difficulty Needed Successes
Simple Failure Routine(+20) 5
One Level of Failure Ordinary(+10) 7
Second Level of Failure Challenging(+0) 10
Third Level of Failure (and beyond) Difficult(-10) 10+Level of failures

How much time?
I honestly do not know how quick one sinks into a slough. To make it a challenging, I would say that a pc sinks not quicker 10cm per combat turn. Adding the fact that there is some kind of “water level” above the “ground” of the Slough at that a humans mouth is not on top of their heads an average human should start to “drown” after 15 to 20 rounds. If he is standing still, that is! Every roll taken should reduce this time by one round. Every level of failure on a role for freeing oneself will reduce the time by another round.

How about help?
Instead of trying to get himself out, friends can try to pull him out. This should be an extended ordinary(+10) strength test and the number of success needed should equal to those getting out by himself. The difficulty can (and should!) be modified by the assumed total weight of the character who got stuck, compared to the weight the helping characters could push (added STB of each of them). Every 5 kg above/beyond the number results into a mali/boni of 1. Take note that the pc stuck can pull himself along if he is handed something else that is “anchored” somewhere outside of the slough. Like a rope around a tree. If the only “anchor” is another pc holding it, the character in the slough cannot pull himself.