[ClearCase_guy]

I had a science teacher tell me something like that back around 1970. I was young and empty-headed and I pictured the scenario as a low-lying dip in the land, with some sort of lingering fog of mustard gas just waiting for an unwary traveler to stumble down the hill and die, 50 years after the fighting stopped. As I grew older I decided that the teacher made the whole thing up (because my scenario was preposterous). But perhaps the tree aspect is more believable.

[KarlInOhio]

I was young and empty-headed and I pictured the scenario as a low-lying dip in the land, with some sort of lingering fog of mustard gas just waiting for an unwary traveler to stumble down the hill and die,

Mustard gas is actually a liquid. It was aerosolized in use. Imagine the droplets from a spray-can soaking an area and then that area remaining poisonous.