tunnelling

Create a tunnel into the ground.

This is the way to build roads underground. It can connect natural caves and towns of underground races. The difficulty of building a tunnel is 4. There are often more units nessecary to build tunnels.
Two units with the same tunnelling skill work togather like a unit with on point more.
When taking the double time to create a tunnel this also gives one point more skill. There is always a tunnel inside a tile with a underground city. It is also possible to move from one underdark tile to another but that takes a lot of trouble without tunnel.