How many skulls are in this game?

I'm perrty sure theres one on each level but i dont know how to get them so good luck.

I just know there R like 15 skulls but I don't know where thy all r. Some of them are really kewl but some of them make things suck. But hear you go.

Level 1 Cairo Station
Go to the B.01 Commons(Look for this sign on the wall)
There will be a ledge where the carts pass through. Try to jump on it. You can use the plants to get a good jumpstart, but you won't be able to hop the fence. Land on the wall lamps then jump again. Once above, go onto one of the slopes holding the rail then jump on the rail. You'll see a trash bin. Knock it off and you'll see a skull called thunderstorm. I don't know it's abilities, but I'm sure you'll be able to find that out.

Level 2 Outskirts
Retrieve two plasma grenades or more then return to wrecked pelican. If you look overhead, you will see that the roof has been damaged and that there are wooden boards sticking out of it. (The goal right now is to get on top of the roof)

Find a good spot on top of the pelican to launch yourself over on one of the boards. I suggest that you get as close as possible to the lowest board because if you don't you will have a difficult time getting up. Place one of your plasma grenades right under you, count for two seconds and then jump. The blast will give you a boost, but you'll also have to crouch to get more air. When you reached the top, cross to the opposite building closest to the courtyard. A mistake in the development of the game's architect will make it seem like you can't get any farther because there is an invisible wall stopping you. Jump on it then look for a dark alley, which at the end of it you will find a skull called blind.

I should warn you though, if you pick up the skull you will have to play the rest of the game with no hud, no reticle, and visual of your weapon until you find another skull or turn of the game console. Level 3 (haven't located skull)

Level 4 The Arbiter
Fly the first Banshee you encountered to the bottom of the forerunner facility. At the center you will find several circular platforms, one of which contains harmless grunts dancing around the skull called Happy Birthday Grunts.
This skull enables you to blow up any enemy if shot in the head.

Level 5 Oracle
After the long elevator ride with flood and the sentinels, there is a room with tons of windows which can be penetrated to advance to the lower level. On the opposite side of these windows, there are more windows which also can be penetrated. To get through it, go on to one of the raised platforms on the lower level and place a plasma grenade under you. Jump and shoot towards the window to get a nice clean landing. To your left are dead manlike parasites surrounding the skull called famine. Don't worry, they are harmless.
I don't know what it's power is.

Level 6 Delta Halo
Go to the third structure where you last meet up with your allies. There are many ledges around the center building that lead to the skull and there are many different paths. The first path starts off near the pelican loading zone.

I won't tell you how to climb up because it will take too long, but I think you'll figure it out. Run along top clockwise until you come upon the pillars. Hop them till you get to the other side. There is a tall ledge and on top two invisible elites. Between them is the skull envy.
It allows masterchief to become invisible for a short period of time.

Happy killing!!