[Someone]

There also still are many, many unexploded projectiles dating back to World War One.

https://en.wikipedia.org/wiki/Zone_Rouge:

“Each year, several tons of unexploded shells are recovered. According to the Sécurité Civile agency in charge, at the current rate 300 to 700 more years will be needed to clean the area completely. Some experiments conducted in 2005–06 discovered up to 300 shells per hectare (120 per acre) in the top 15 cm (6 inches) of soil in the worst areas.”

https://en.wikipedia.org/wiki/Iron_harvest:

“The iron harvest (French: récolte de fer) is the annual "harvest" of unexploded ordnance, barbed wire, shrapnel, bullets and congruent trench supports collected by Belgian and French farmers after ploughing their fields. The harvest generally applies to the material from the First World War, which is still found in large quantities across the former Western Front.”

These also fairly frequently include unexplored poison gas shells.

Belgium als still has a few big mines with thousands of kilograms of explosives near Messines that may one day go boom. One of them did in 1955 (https://en.wikipedia.org/wiki/Mines_in_the_Battle_of_Messine...)

[cafard]

American University in NW Washington, DC, had an army chemical warfare station during WW I. After the war, they suitably disposed of the remaining ordnance, according to current protocols: they dug holes, dumped in shells, and buried them. The area where this was done includes some of the most expensive real estate in Washington. Years ago one would read of old mustard gas shell unearthed at the South Korean ambassador's residence.