The aim of the Watching series is to draw attention to some of the very interesting items around us, things that perhaps we don't notice as much as we might. The first was Bridge Watching, and when this was put ''on the Net'' it produced, to the surprise of the author, such a pleasant flood of e-mail that another was written, called Water Watching. This, too, was kindly received. So it was tempting to continue with the theme. Some people find pleasure in taking photographs and some like to sketch or paint, because tunnel mouths are often set in lovely countryside. A train emerging from a hole in the side of a hill makes a good picture, in any weather conditions, whether as a photograph or a painting. On the other hand some people like to just look, without recording the sight. Whichever may be your choice, I wish you happy tunnel-watching. There are other reasons for tunneling, such as providing an approach for men and materials, to get at something not otherwise accessible from the surface, escaping from a prison, robbing a bank, or following a seam of mineral deposits, perhaps to carry water or other liquids from one place to another. As you extend your knowledge during tunnel-watching sessions I wish you well. You may also be drawn to some of the literature on the subject, either easy introductory material like this or more technical treatment, leading you along fascinating lines of learning. Go forth and enjoy this free entertainment.