WWI underground: Unearthing the hidden tunnel war

One of the largest archaeological digs on the site of the Battle of the Somme has just begun.

The first day of the battle was the bloodiest in the history of the British Army with 20,000 men killed and 40,000 captured or wounded.

Archaeologists have begun digging into a large network of tunnels and trenches beneath the battlefield.

The BBC's Robert Hall reports from northern France.