Season 1 Episode 6 is the iron rig where earth benders were held captive. Doing a glance through of season 1 and 2, I don't see any other episodes like that.
I certainly don't recall an episode with a bunch of earrthbenders being held in a valley. The closest is King Bumi being held in his own city where he could break himself out of at any time.
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The batman villians all seem to be one big joke that batman refuses to laugh at - SantaRPG