Uh, this is frustrating!

We've found the two first tablets and - as far as we can see - we've explored the entire Lost City within getting the Third One (so I guess we haven't been everywhere yet ). We've opened all the doors in the roundabout thingy and clicked on the levers in the two rooms behind the "shell room" (where the Mother of Crabs got a beating), but nothing seems to have changed. What are we doing wrong? Could someone please point us (gently) in the right direction?

Oh, and we took care of that deluded "mermaid"/dartha eel creature (at the time we thought that that would have led us to the third tablet, but noo...).

*Sigh* It's a great game, it's just so annoying that we get stuck like this rather often. Oh well, I hope someone can help us here...

Almost all of us needed help sooner or later, so don't worry.
You're very close to the third tablet, you only need to find out a way to make the lever STAY turned. Since this is not that obvious I'll tell you how:
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you'll need to fill both keyholes with glowstones at the same time. But since you won't have a glowstone to spare, I'll leave you the puzzle of how to accomplish that. (There are no more than 3 glowstones to be found in the dungeon.)

Thank you, you are a life and marriage saver! I don't think we would have figured that out in a million years (or thereabouts)

Onwards then!