The door comes open and the party enters. The hallway here is broken and crumbling. The stones are chipped and crushed between one another as the weight of the land surrounding them seems to be pushing in ever so slightly. In certain areas the stones are missing, revealing a depth of rock and soil behind, with much of it spilled across the cobbled floors. The hallway bends left before long, and seems to take an uncomfortably steep descent after that. The slope here is not a natural part of the architecture, but rather it is likely a shifting of the structure within the ground. It is steep enough that caution must be taken to not lose balance. So long as you are not under pressure, you feel, it should not be an issue to stand upon.
Shortly down the slope, just 5 feet after the left-hand bend is a cave-in of sorts. The ceiling and right wall have collapsed and a large mass of stone and dirt have spilled into the hallway, nearly choking the entire space. The torchlight, however, shows the area that Zubie said one could "squeeze past," and he apparently meant those words literally. On the left side of the blockage, closer to the ceiling than the floor, is a small gap that looks like it would lead behind the cave-in. It is only perhaps two feet wide; three at most.
OOC:
No need to make Acrobatics checks to squeeze through, since you're able to take as much time as you'd like in order to succeed (if you have to do this during combat, it'll be a check). However, you cannot squeeze through with your backpacks on. So even though we aren't in rounds, simply make a mental note that to go through this obstacle you have to remove your backpack, crawl through, and then drag the backpack through behind you before putting it back on (or whatever way you figure to do it).