The Red Zone is a series of areas in northeastern France that were isolated by the government after World War I. These areas were too damaged by the war for agriculture or human habitation. Some small areas remain off limits.
Each year dozens of tons of unexploded shells are recovered and, according to the Sécurité Civil agency in charge, at the current rate no fewer than 700 more years will be needed to completely clean the area. Some experiments conducted in 2005-06 discovered up to 300 shells / 10.000m² in the top 15cm of soil in the worst areas.Some of these areas have been set aside as artificially planted state forests but the trees are in poor shape. Other areas have been cleared for agriculture but concerns about pollution remain. Much more detailed information can be found on the French language wikipedia page.
Some areas remain off limits (for example two small pieces of land close to Ypres and Woevre) where 99% of all plants still die as arsenic can amount up to 17% of some soil samples. Most destroyed villages lie in this area.