WotC has published an article by Shannon Appelcline which delves into the origins and evolution of the Underdark, from its first appearance in Greyhawk, in Gary Gygax's Hall of the Fire Giant King (1978), to when Douglas Niles named it in 1986's Dungeoneer's Survival Guide, up until the present day Rage of Demons storyline for D&D 5th Edition. The article also covers - of course - that most famous of Underdark denizens, the dark elf Drizzt Do'Urden. Did you know the Underdark was known as the Deepearth before it was the Underdark?
Click on the pretty Underdark art to take a look!
Click on the pretty Underdark art to take a look!