Thank you for the reply!
Your method is pretty effective, I also tried the sweep tool which works well.
Now my problem is that I would like to connect the tunnel to an other surface as shown in the attached picture.


I have been trying all day to find something, but with little success. I tried different things including bridging the two surfaces but I did not manage to get a smooth transition from one surface to the other.
Is there a way to blend the two surfaces?
Any help would be really appreciated!
Marilu