The far side of the third chamber looks the most interesting. Comparing this with the drawing posted by wysefool, we can see that this is the porthole entrance mentioned by rhiannon.
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Baza you stonehunter extraordinaire, it looks just like that drawing. So did you lean in and see if you could feel the buried porthole? Or was the whole thing all full up with stuff I suppose? Or were you afraid something might grab you haha.
Baza, we are not worthy. Well done indeed. Definitely worth the looking, too.