As someone who frequents valleys, I was unhinged to learn of Dax Cowart's calamity beneath.
What types of alfresco terrain ought I most beware? It obviously can happen only in small hollows in the land (that I'll call dale), but how small?
Please see the titled question. I know that propane is denser than air.
In July 1973, Cowart, then a pilot in the Air Force reserve, and his father were visiting a tract of land that his father was thinking of purchasing. The land lay in a small valley and, unbeknownst to the Cowarts, a gas leak had caused the valley to become filled with propane gas. After surveying the land, the Cowarts returned to their car, and the sparking of the ignition set the gas on the floor of the valley ablaze, severely burning both men. According to Cowart, "I was burned so severely and in so much pain that I did not want to live even in the early moments following the explosion. A man who heard my shouts for help came running down the road, I asked him for a gun." He said, "Why?" I said, "Can't you see I am a dead man? I am going to die anyway. I need to put myself out of this misery." In a very kind and compassionate caring way, he said, "I can't do that."
Afterword: I learned of this on p. 63 of Better Never to Have Been (2008 1 edn).