I have mostly read and heard of tunnels as an offensive tool in the Somme offensive to place explosives near enemy positions without being exposed to enemy fire across no man's land. This book on geology and warfare has a chapter on the Western Front that describes some of the aspects of trench warfare and tunneling.

There's also this BBC video and article that is less dry in it's presentation when talking about tunneling.

Miners would not only have to dig tunnels to place explosives, but watch out for signs of enemy digging and be wary of "counter-mining" while underground.

Hopefully someone can expand on counter-mining as a defensive practice.

As for the trenches question, they would usually have a zig-zag pattern instead of straight lines. If you were looking at an overhead picture, what you might be looking at would be trench lines leading towards rear areas as you wouldn't want to have to run above ground to the main or support trenches from rear areas. Alternatively, there were also listening posts that would jut out into no man's land from the main trench positions, which might be what you're thinking of.