There are several keys in the game to collect, here are their names and instructions on how to obtain them:
 Key of the Captain
Obtained by: Going into the dead sea monster's mouth at the beginning of "The Hydra Attacks" level. Inside is a man hanging off about to fall into the sea monsters stomach. The cut-scene shows Kratos grabbing the key and letting the man fall.
Used for: Opening the door to the next part of the level.
Obtained by taking the key from a woman that fell from a building.
Used for: Opening a locked hatch.
 Muse Keys
Their are 2 muse keys. They can be obtained by finding each one in a chest somewhere in the game.
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