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That cellar in linked to a quest in the DLC Broken Steel. This quest in given to you have the main questline in completed (the DLC lets you keep playing) This quest is named Holy Water (YOU WILL ONLY GET THIS QUEST IF YOU DID NOT DESTROY MEGATON) But to answer you question the key is found on a man named Brother Gerard, If you didnt destroy megaton, asked scribe bigsley at the Water Purifier for the quest, asked around megaton about the cult, then you should see this guy hanging around the cellar, he has the key.

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15y ago

What else can I help you with?