C386 Three Caves

The symbol on the third cave was not a pattern, but a symbol. The symbol was similar to the word "Mi", except that one side seemed a little thicker. The other seven were about the same size, except for one that was thicker. What did this thicker line represent?

Then he looked at the map in his hands. The top only had this thick line, as if it was a casual drawing at the side of the cave