There are several theories about it: Buddhist rituals, resurrection...
It's a fact that one of the most perturbing items was a 1956 Atlas Mystery Tales #40 comic, in which cover appeared a man completely freaked out by the vision of a city in the clouds, floating above another city. Another thing to note from this cover is the almost invisible text in the bottom of it that reads Airlines. So we know the guy is looking through a plane's window.
Maybe you're one of those people who says this is only a mystery story for kids, that it's not relevant in the Lost plot. Are you really sure? Nothing in Lost is coincidential.
Let me show you something. Here you can see the Lost 4th season promotional poster, with all the main characters from our favourite island in it, surrounded by the great and deep ocean. Do you see anything weird on this image? No? Let's take a closer view of a part of it, or even better, lets put it upside down, and see what happens with that big chunk of sea water on the lower half of the image. Actually, let's put it side by side with the Mystery Tales comic cover.
They looks almost the same! What is the relationship between a 1956 old fashioned kids comic and the promotional Lost image? Are they trying to tell us that the island is also hidden? Is that the reason because nobody (until now) is able to arrive to the island?
Too many questions, and only 3 episodes left from this 4th season.
Everything points out to this: we need that comic.