The Call of Afeon
A network of tunnels connecting all of the dwarf cities, from the smallest village to the major metropolis of Black Rock Mountain. Being the masters of engineering that they are, the Dwarves have created a system of flooded tunnels to make sending heavy materials and people to other cities quickly and with less work. The departure area for each of these tunnels is several stories higher than the destination area at the paired city. A large, flat-bottomed boat is floated down the resultant underground river, guided along the tunnel by metal rails set into the walls. To move the boats back up to the departure level, an underground loch is used. A pair of these tunnels don't need to be fed much water, since it can be recycled indefinitely.