Ruby Falls has one of the highest (if not the highest) underground waterfall in the world. I think it's pretty neat.
Near Sweetwater, TN (up I-75 toward Knoxville) is The Lost Sea. It's the world's second largest underground lake, it covers a few acres I think. When your group gets down to it, they take you in it in boats! It's got a lot of lighting installed so you can see everything. Also worth a possible stop.
There's way too much to do in Atlanta to list here.
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