There isn't a dwarf alive who remembers the Deep Roads as they once were. They were the network of tunnels that joined the thaigs together. To be honest, it isn't even right to give them such a simple term as "tunnels": They are works of art, with centuries of planning demonstrated in the geometry of their walls, with the statues of the Paragons that watch over travelers, with the flow of lava that keeps the Deep Roads lit and warm. The cloudgazers up on the surface talk of the Imperial Highway built by the magisters of old, a raised walkway that crossed thousands of miles, something that could only have been built by magic. Perhaps it is comparable to the Deep Roads, although we dwarves didn't need magic.