Awesomely frightening, Underdark finally makes the classic adventuring area truly chill inducing. The Underdark has always been filled with potential, but its actual depiction has traditionally been a let down. Generally it just boils down to caverns, more caverns, and look over here: caverns. Even 5e suffers from this problem (see Out of the Abyss with the exception of Zuggtmoy’s Wedding).
This 4e book portrays The Underdark as a diverse world - each area more terrifying than the last. It truly shows a locale that should be avoided by any sane sentient while providing important reasons for adventurers to leave their better judgement behind and descend into the depths.