This tunnel is 200 miles long connecting the Citadel with Stahlwerk and Stahlwerk with Svätyne, it original plan was to eventually also connect other Major cities and even the Dwarf City.
It’s initial progress was very slow due to the hard rock (350 feet of solid rock) that had to be moved apart with stone shape or softened into clay with soften earth spells.
Eventually a Drill was made to help with the progress. After getting through the citadel’s hard rock (which took about 2-3 months)and ascending to a depth of about 60 to 70 feet underground, (right on top of the hard rock layer), the earth was pushed by compression and some mining to make a tunnel. Think of a huge sized dire badger leaving the typical tunnel behind him (look at dire badger description here), but larger. Occasionally rock formations that rose complicated the operation, normally the tunnel would be angled upwards or it’s path slightly changed, occasionally the rock had to be cut.
Cubic feet of hard earth 110,490,000 ft^3
Cubic feet of hard stone 10,200,000 ft^3
Amount of hard stone removed with spells 5,606,400 ft^3
Amount of hard stone crushed with drill 4,387,680 ft^3
Rate of drill trough stone about for the first 4 months only 8 miles per year but after the upgrade 1.6 miles per year.
Rate of drill through soft earth about 50 miles per month.
Amount of wooden planks that should be put in the tunnel for it to be safe 2,112,000
Planks actually put to support 1,425,000
Volume of wood cut and processed by hydra while at the citadel 392,699ft^3 about 1500 large trees(about three acres where cut).
Amount of planks actually made 2,137,500