tube shaft

In a similar manner to the deep-level London Underground stations, each track and platform is in a separate tube shaft with a common entrance and exit point between the two tubes.

The thistle tube shaft is designed to allow insertion through a small hole present in some stoppers, permitting the tube to be inserted into a container such as an Erlenmeyer flask.

The British used tube shafts from May 1915, a full year before the Germans, who when they did start to use metal and concrete tunnels, had lost the strategic advantage and were digging purely for defensive purposes.