As you ascend into the next level of the cavern, the area is permeated with a rather thick roiling fog of dust and moisture. It has very unusual properties. The mixture of fog is roughly a foot high, but every step kicks the stuff up into the air. It's almost as if the fog is lighter than the air, and is being "pulled" towards the ground by some force other than gravity. It roils around in swirls at points, but is generally very calm.
Up ahead, it is completely dark. Shadowy illumination permeates the next 30 feet of the corridor, which is now only seven and a half feet wide and high. The rock appears to have been torn very quickly, and there are still loose bits of stone throughout the walls and even the ceiling (though the floor is hard to see, it probably has the same properties).
[I need to know about light sources for those without darkvision, and perhaps whether or not your using cover for your breathing; any other notes such as if the group is moving using move-silently or searching the corridor as they go should also be noted.]