The Heart of The Island
I think enough time has passed since the finale that it’s safe to post spoilers, so the first order of business is to identify the location of the Heart of The Island. Jacob gave Jack clear directions on how to find it: the “source” is over a ridge, just beyond the bamboo grove where Jack first woke up in the pilot episode. We know that the bamboo grove is very close to the fuselage crash site, since Jack was able to run from the grove to the plane wreckage in a matter of moments. Therefore, the Heart must also be close to the southern coast, just a short hike from the survivors’ beach camp. Its location is indicated by the gold starburst northeast of the fuselage. Even though it’s this close to the beach camp, the survivors wouldn’t have discovered it since only the protector of The Island can find it.
After watching the final episode, it is also obvious that the cliffside cave is not located along the northern shore of The Island. The Man in Black and Jack reached the cliff far too quickly for it to to be at the extreme other end of The Island. Based on the appearance of the terrain, the cave is most likely located south of the volcanic crater. We know that, although they frequently explored the southern areas of The Island, the survivors never discovered the cave, which would indicate that they must have found another path around the crater, or that the ladder is situated in a way that makes it difficult to see.