The Under Roads

Built by the dwarves long, long ago to connect their underground empire, the Under Roads are a massive network of subterranean tunnels that span across all of Aurend and Wracias and, if rumors are to be believed, even go under the ocean and connect the two continents.
Dwarven chronicles have no records of when the Under Roads were built, all that they know is that it was a very, very, VERY, long time ago. Some believe they were built during a time before dwarves even emerged onto the surface. Many of the tunnels have fallen into disrepair and 90% of the Under Roads are no longer used because they are simply too dangerous, as horrors from the deep have burrowed their way up and goblins and orcs from the surface have burrowed their way in, not to mention that they are almost impossible to navigate for any great distance without a recent and up to date map.
Entrances can sometimes be found on the surface as the crumbling tunnels collapse, opening up passageways as the surface caves in. Imperial authorities patch and fill these holes when they can, lest some monstosity emerge and start butchering the local peasantry. They can't always find them all however, and enterprising adventurers seeking to make it big mount expeditions into these surface entrances hoping to stumble across some lost dwarven thaig or ancient artifact to sell on the surface. These entrances are hard to find however and constantly change due being patched or caving in even more.