The Caves are the highest of all of the regular areas, I'm not counting the 5 mini-areas even though they have higher cost Bushes.
The Maps 'show all areas' should be showing you 16+ squares on the main map that you don't have access to unless you have already done them. If so then you should know where they are. But they start at the very last square of the Jungle, The Heart of the Jungle.