The World of Darken
The Dwarven Nation of Thangar is mostly underground. The known fortresses are all carved from mountainsides and linked to each other by huge underground tunnels known as the Dwarven Highways. Actually they are two tunnels that run straight as an arrow between two of the fortresses. The two tunnels cross each other at an intersection in the middle of the nation. Although these highways are known publicly, no non-dwarf has ever been allowed to use them or even see them.
Characters from Thangar gain a +2 to dungeoneering checks, and a +2 to perception checks