Below the center of Arras, there is a whole network of subterranean corridors, called Les Boves. The corridors are old chalk mining dating from the 10th century. Later on, these caves were used as grain warehouse during war times. During WWI, these corridors were of strategic importance, as demonstrated in 1917 during the Battle of Arras. 24 000 British troops were hidden in these corridors. You can still visit tehse corridors. One of the main access points is the Town Hall.