It's a lot more widespread than most people think. A quick look at Wiki yielded this:
https://en.wikipedia.org/wiki/Unexploded_ordnance
France and Belgium-
"In the forests of Verdun French government "
démineurs" working for the
Département du Déminage still hunt for poisonous, volatile, and/or explosive munitions and recover about 900 tons every year. The most feared are corroded artillery shells containing chemical warfare agents such as mustard gas." (That's mostly WWI ordinance)
Laos-
"Some 288 million cluster munitions and about 75 million unexploded bombs were left across Laos after the war ended. From 1996–2009, more than 1 million items of UXO were destroyed, freeing up 23,000 hectares of land. Between 1999 and 2008, there were 2,184 casualties (including 834 deaths) from UXO incidents."
and the list goes on...
ones the other side left behind.
...but no matter where it is, it's not always the 'other side'...
"According to the Department of Defence, "millions of acres" may contain UXO, Discarded Military Munitions (DMM) and Munitions Constituents (e.g., TNT or RDX).
According to US Environmental Protection Agency documents released in late 2002, UXO at 16,000 domestic inactive military ranges within the United States pose an "imminent and substantial" public health risk and could require the largest environmental cleanup ever, at a cost of at least US$14 billion."
Close to home (discussing a Canadian Army artillery range on the edge of Calgary)
"Areas long considered cleared can still yield unexpected dangers, and even areas that had never seen live fire can be affected. In the Canadian Government’s own words “UXO can also move or be exposed over time. For example, freeze-thaw cycles, flooding and storms can uncover buried ordnance or move it from place to place. Just because no one has seen UXO in an area for many decades does not mean that it isn’t there now.”
https://calgaryringroad.wordpress.com/2013/09/05/unexploded-ordnance-in-southwest-calgary/
It's not just a modern phenomenon either...
"Although comparatively rare, unexploded ordnance from the American Civil War is still occasionally found and is still deadly 150 years later. In 2008, for example, Civil War enthusiast Sam White was killed when a naval shell he was attempting to disarm exploded."
Tread lightly...