An estimated 720 million shells were fired during the Great War, with approximately 12 million failing to detonate. At places like Verdun, the artillery barrages were so overwhelming, 150 shells hit every square meter of the battlefield.
Though the Zone Rouge started at some 460 square miles in size, cleanup efforts reduced it to around 65 square miles. With such massive amounts of explosives left in the ground, the French government estimates the current rate of removal will clear the battlefields between 300 and 900 years from now.
Source: After 100 years World War I battlefields are poisoned and uninhabitable