Moxie and Misto get acquainted with a short go-to-ground tunnel yesterday. The tunnel is a little worse for wear -- equipment ages as I do, and this one is a replacement for two others that have gone the way of all things.
Go-to-ground tunnels are useful for training pups that are far too young for the field. They get used to going in and out of dark spaces, and they discover awards inside (initially food, and later quarry).
For a simple how-to on tunnel training terriers, see this 101-piece. Serious diggers, of course, are advised to get the book, the cheapest and most useful bit of terrier kit you will ever own, as it will pay for itself on first read.