Why was there Trench Warfare in WW1?

In the first of our new 'how and why' podcasts we ask a simple question: Why was there Trench Warfare in the First World War? What factors made it possible, where were the first trenches, who dug them and how did they affects the battles in WW1?Thanks to Doug @colour_history on Twitter for the use of the colourised image of men from the 1/4th East Lancashire Regiment in the trenches in January 1918. Got a question about this episode or any others? Drop your question into the Old Front Li...