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THE SIMPSONS: When the Simpsons discover a collection of keys to every door in Springfield, Lisa stumbles upon an eerie hidden classroom beneath Springfield Elementary School in the all-new "500 Keys" episode of THE SIMPSONS airing Sunday, May 15 (8:00-8:30 PM ET/PT) on FOX. THE SIMPSONS ™ and © 2011 TTCFFC ALL RIGHTS RESERVED.