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Halo 3 Walkthrough Cleansing - Infinite Devil Machine
At the bottom, go to the right. Now the Gravemind is talking to you.
Keep going until you reach another hole down.
Go to the right and follow this passage. When you enter the room the Gravemind talks again.
The glowing blue thing in the back has Cortana. Walk up to it and hold RB to receive a message from Cortana.
This brings the Chief to the end of the 5th mission.