Underway

The Underway, or Ungdrin in the Dwarfen tongue is the name given to the great inter-continental highway of tunnels that had at one time, stretched the whole entirety of the World's Edge Mountain, from the frozen peaks of north to the lush jungles of the south. In the great days of the Dwarfen Empire, all of the greatest strongholds were all linked by this complex system of tunnels, which all connect to one singular main highway that is titanic in scale. From this main highway, there lies a multitude of smaller tunnels that branches out to a variety of different mines, forts, watchtowers and outlying Strongholds within the World's Edge Mountain.

In modern times, the Underway has since fallen to disrepair as a result of the ensuring catastrophe brought about during the Time of Woes and the Goblin Wars. The Underway was pivotal to the success of the invading Greenskin and Skaven armies. Each of the finely constructed tunnels allowed the enemy to bypass treacherous terrains that would've taken months to cross or even years to carve out from bare rock. With these access points, the armies attacked the Dwarfs at their most weakest point; from below. Since those horrific times, the Underway is mostly devoid of activity save perhaps for the occasional enemy armies that have since used the highway as a means of travel.

Today the Ungdrin is completely unsafe. Not only is the structural integrity of the tunnels are constantly ridden with rockfalls and cave-ins, but its lenght had since been broken up by large chasms and pits that were created from earthquakes during the Time of Woes. Monsters of all kind lurk in even the broadest and most structurally sound sections, with a significant population of Skaven and Night Goblin tribes also taking residence. In more recent years, Dwarf expeditionary armies have managed to clear and repair a few of these tunnels to help link the scattered Holds of the Dwarfs together. Although dangerous, the Dwarfs have recently began to use these tunnels once more, although they only do so in large numbers and fully armored.

Source

 * Warhammer Armies: Dwarfs (5th Edition) -- pg. 18