In 1916, the British linked the existing chalkytunnels, caves and quarries at Arras to create a safe undergroundnetwork, dug by tunnelling companies from Britain and New Zealand.
In April 1917, 24,000 men who had been hiding in the tunnels attacked. The British advanced about 8 miles.
However, their progress slowed and by the end there were nearly 160,000 British and Canadian casualties.
