Excavating tunnels from World War I
3 November 2011 Last updated at 07:07 GMT
Four miles of tunnels which were dug during World War I in the Somme battlefield are now being excavated by a team of British and French archaeologists.
Some of them are up to 100ft (30m) below the ground. Robert Hall is in the village of La Boisselle, in France, with the details.