Well, they do give you two clues that this is an unusual area. First, it's the only place where there's a lava flow that leads in the direction you need to go and is the only lave found in a tunnel like that (yes, there is lava in other tunnels but those tunnels also have floors in them to walk on where this one doesn't). Second, when you first approach the area there is an earth tremor, the only one in the entire game. Those two together should be enough to give most people the hint if they really think about it. Heck, compared to those darn maps in Wiz 7, this was practically a giveaway.
I remember spending days and sometimes weeks trying to decipher the meaning of a map brainstorming with friends and so on. I believe it was the Crystal map that we found to be the most difficult but even the Temple map was a challenge the first time through. And that darn well riddle in Munkharama!