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The unlockables in the game:

[edit] Modes

[edit] Extreme/European Extreme Difficulty

Complete the game on any difficulty level to unlock Extreme difficulty. Depending on your version, European Extreme might also become available.

[edit] Boss Survival and Casting Theater

Complete the game on any difficulty level to unlock both. They aren't available n the US version of the game.

[edit] Items

[edit] Bandana

Gives you infinite ammo when you have it equipped. It can only be used in the Tanker Chapter on any difficulty. You need to acquire a reasonable number of dog tags to acquire it (around 30 in the US version, 46 in the European version).

[edit] Stealth

When equipped you are invisible to all enemies, even cameras. You can earn one on the Tanker Chapter, and one in the Plant. For the Tanker chapter, you need around 60 dog tags (US) or 76 (EU). For the Plant you need around 120 or 121 (again depending on version).

[edit] Wigs

[edit] Brown Wig

Gives you infinite ammo when you have it equipped. It can only be used in the Plant Chapter on any difficulty. You need 73 dog tags (EU) or 80 (US) dog tags from the Plant.

[edit] Orange Wig

Gives you an infinite grip meter when you have it equipped. It can only be used in the Plant Chapter on any difficulty. You need around 160 dog tags (US) or 170 (EU) form the Plant.

[edit] Blue Wig

Gives you an infinite supply of air when you have it equipped. It can only be used in the Plant Chapter on any difficulty. In the EU version, 218 dog tags from the Plant are needed. In theUS version, all of the Plant's dog tags are needed.