To help keep the water in, the Trust has installed several miles of polythene sheeting. This is called a membrane and sits vertically in the ground up to a depth of 4 metres, right down to the underlying clay. This keeps the water in the Fen, where it is needed. So far they have been installed on the northern side of the Fen, and around Priory Farm to the south. They are very expensive to put in and it can be a very difficult job fitting them. Since they were fitted, the water levels have been better. However in recent years, rainfall has been low and areas can still dry out.
