In the tunnels underneath a WWI battlefield

Archaeologists are beginning one of the largest investigations ever mounted of the tunnel system underneath a WWI battlefield.

Some of the tunnels under the Western Front battlefield at La Boiselles, known to British troops as the Glory Hole, have already been opened and are remarkably well preserved.

The site had remained untouched since the war ended, with the remains of 28 British tunnellers entombed in the ground where they died during military mining battles.

Robert Hall went underground with military historian Simon Jones.