Quote from Aiine >>

For the Underdark there is some good stuff in the 3rd party space for 5e. AAW games put out a 3rd party adventure called Rise of the Drow. If you're looking for an Underdark sourcebook this is def something to check out. Also there was just a kickstarter for the Survivalists Guide to Spelunking 

Thanks for this.  My take has been either after or maybe in place of the classics revisited, WotC should publish some world building tools a la Dungeoneer's Survival Guide and Wilderness Survival Guide from AD&D.  Yes both are largely about making under dark and wilderness more interesting to players, give the environment more to the game than just scenery for encounters, but both had pretty substantial guides on how to design an Underdark from the "realistic" geology one would have to traverse to get to it right down to things and features that literally never see the light of day.  The Wilderness Survival Guide does the same for designing continents that are in line with realistic geographic features.  

The fact that the lead writer for the Dungeoneer's Survival Guide is writing a 5e equivalent via third party is still pretty cool.

53 goblins are attacking people.

I agree with people who say it should be an Underdark sourcebook instead of an OOTA sourcebook.