It's safe to remove the "I think" dependent clauses, Andyr: The first one is indeed (pretty much) as close as you can get to the thing before the boulder suddenly appears, and the second one is after it has done so. The green "party go here" markers are in exactly the same position in both screenshots, by the way.
The ironic thing is that, whatever the thing is, it looks
more like a cave when it's blocked than when it's relatively accessible: The boulder blocking the 'entrance' is clearly hollow, and there's light shining from beneath it, making me really really want to know what was denied to me.
So if, indeed, there's an unused ARE file that looks rather Hellish, this might be just where it was originally intended to go.