Hey, thanks for the help! There was, indeed, a lever that I couldn't see. It was actually dumb luck that I found it this time... I just happened to click on that space... hehehe.
I wonder if one is more likely to miss the lever depending on their computer-- my powerbook screen tends to dim of its own accord without me noticing (something to do with my power save settings), making it even harder to see underground.
Now let's see what happens...
thanks again, folks!