In the last section of the caves, right at the end of the passage where you turn left and meet Dagoth Gares, the tunnel dead-ends.
The end of the tunnel isn't a wall but an impassable pile of boulders.
I can't fit through but I can look through the gaps in the rocks and there is definitely some sort of chamber back there, the map also hints at a wider room beyond.

After I had finished with Gares and his pals, I spent quite some time trying to figure out how to get in there, but there doesn't seem to be any way around the obstruction.

Anybody know what the deal is?