Baldur's Gate : The Sword Coast Chronicles : NWN2 Persistent World

A map of the Underdark, for reference (both IC and OOC; with corresponding Faerun surface map).

Please note that although this map is posted in the Lore of the Lands thread, it would be very unlikely that an Underdark Player Character with a natural Lore skill rank less than 20, and a Surface Player Character with a natural Lore skill rank less than 30, would have attained the geographical knowledge that is on the above Underdark map. Nonetheless any knowledge of lands, areas and links between locations on the above map would take some actual Role-play gained knowledge, or direct experience.

Granted, there might be extenuating circumstances that would change the minimum Skills...but that is for a DM to decide.