In Faerun, Underdark is the home of the Drow. What is its equivalent in the lands of Flanaess? Is there also some sort of Underdark there, or is there something else?
Although the Forgotten Realms popularized the Underdark and its role as home of the drow, the Underdark actually pre-dates that setting1, and canonically it forms the home of the drow in the World of Greyhawk setting too.
According to the Living Greyhawk Gazetteer, many entrances to the Underdark are located underneath the Hellfurnaces, a volcanic mountain range which marks the border of the western Flaeness:
The steep walls of the Hellfurnaces are reliably reputed to hide many cavernous entrances to the Underdark.
The drow in particular are among the Underdark's inhabitants beneath the Flanaess:
Places here include the infamous Vault of the Drow (a cavern under the Hellfurnaces reported to contain the drow-ruled city of Erelhei-Cinlu), and various underground cities reportedly populated by mind flayers, kuo-toa, wererats, troglodytes, derro, duergar, aboleth, ghouls, albino humans, bullywugs, etc.
The D&D 3.5 modules Expedition to the Ruins of Greyhawk mentions the drow of the Underdark:
Open warfare tore through the Tower of Magic as the conflicts between the factions escalated and grew bloody. Some factions opened tunnels to the Underdark to forge alliances with drow, kuo-toa, and other evil denizens.
According to the World of Greyhawk wiki, the Underdark beneath the Flanaess is sometimes called the "UnderOerth". I can't find any canon reference to this term (Greyhawk-specific sourcebooks simply call it the Underdark), but it seems to occur in some of the regional Living Greyhawk modules.
1 The Forgotten Realms Campaign Setting first released in 1987. The Underdark and its description as home of the drow appeared in the Dungeoneer's Survival Guide (1986), and its existence was hinted at in the D-series of modules as far back as 1978. Although I've seen it asserted that the Underdark began as a World of Greyhawk element, it appears more reasonable to say that it began as a generic setting element which was used by both Greyhawk and the Forgotten Realms, much like how Moradin appears as a god of dwarves in both settings.