In the newest snapshot (14w30a/b), when you go into spectator mode, and fly under the surface, you can't see any cave, underground structure that you built, creeper or anything...
Previously, i was thinking that spectator mode is not only for spectating people on minigames servers or x-raying treasure rooms and caves. I used spectator mode for looking at the cave gen. algorythm (well yeah.. sounds weird) and i used it to solve some issues with redstone in my test world and stuff.

Now i can see underground stuff, that is only 1 chunk in front of me... I think that this is not a feature, that it is a bug... A big bug.

(sorry for any mistakes, if you find some, please write down that there is a mistake. I don't want to damage your eyes )