A fire in the Sunshine silver mine near Wallace, Idaho broke out underground, filling the working levels with smoke. This smoke, from a fire that started back in a worked-out section of the mine that had been sealed off, contained carbon monoxide that ran as high as 20,000 parts per million.
400 parts per million is considered a fatal dose.
A short-circuit in the ventilation system circulated this bad air through the entire mine in a matter of minutes and 91 men were killed before they could be brought out. Two men survived, trapped next to a fresh air borehole 4,800 feet underground. They were found a week later.
I just finished a book on this fire and the aftermath entitled The Deep Dark:
Disaster and Redemption in America's Richest Silver Mine. I recommend it.
I also came upon this article, written years later, by Bob Launhardt, the safety manager at the mine on May 2, 1972. Launhardt was one of the heroes who first entered the mine to try to save the others His article is long and technical but it makes for interesting reading, especially when he describes first-hand his own experience in the burning mine. He didn't have to go in there after the fire broke out, but the fact that he did says a lot about the kind of man that he was. The first responders who went into the towers on 9/11 would have understood. But Bob Launhardt made it back out and he's spent decades trying to understand and explain what really happened that day.
No one still knows for sure how it started, but what is known now is that little mistakes were made in the operation of that mine and these little mistakes combined suddenly one day and created 77 widows and over two hundred fatherless children in the Wallace and Kellogg communities.
91 men in one day, the cost of silver. And it wasn't even the worst mining disaster in American history. If we measure by the number of dead, it was only the seventh-worst.