Just a quick review. I agree with pretty much everything Rachel B. said, and the fact that you get all these unique environments and ideas plus 7 soundscapes...very good bargain. If you use the Forgotten Realms campaign setting you can plug these environments into a new (undiscovered) large section of Undermountain, or even underground areas in Chult or Maztica (though perhaps swap out a few of the inhabitants for those areas). It could be a pocket dimension or the beginning to some other plane of existence. There's plenty more ideas you'll discover on your own as your read through this unique accessory, and I recommend giving it a try.
[5 of 5 Stars!]