No later than the 3rd century did men started digging the limestone plateau in order to take shelter from the Norman invasions. They developed a 3 km long network of galleries and chambers they called " muches " (hiding places in the picard dialect), so that a real city spread all over into a complex of 26 galleries and 300 chambers which could accommodate up to 3000 people along with their livestock. Public places, stables, wells and chimneys can still be identified nowadays.