Some have been preserved as historical sites, such as the Verdun sites listed above, or the Beaumont Hamel Newfoundland Memorial , which remained from the Battle of the Somme. There’s also the giant Lochnagar crater near there that is a memorial site.

This article from the Atlantic has some great pictures of WWI sites and memorials and touches on some of the complications of cleaning up the area. There were a LOT of unexploded shells in the area (because there was so little movement on the Western Front, the trenches were really in a pretty concentrated area between the Swiss border and the North Sea.) They (farmers especially) still unearth unexploded shells, and the French government employs dedicated disposal experts.

Jay Winter, a well- respected WWI historian has written a great book about the meaning of WWI memorials, called Sites of Memory, Sites of Mourning: the Great War in European Cultural History. It’s about the discourse around the collective memory surrounding the war and its impact on the first half of the 20th century. How and why communities memorialized the war said as much about the present as the past.