Darby No. 1: five men in a small mine in Harlan County

HOLMES MILL, Ky. — The explosion came early on the morning of May 20, 2006, at Darby Mine No. 1, a small coal operation in Harlan County with a crew of six underground. Five miners were killed. One survived and walked out.

Coming four months after Sago and Aracoma, Darby completed the worst stretch for American coal mining in two decades, and it made a point that the two larger disasters had not: the risk is not confined to the big mines. Small operations, thin crews and short budgets carry the same methane and the same rock.

The federal investigation looked at ventilation, methane monitoring and the mine's plan for both. What it found was a mine that had been worked outside the controls its own approved plan described, with no one on the property whose job was to catch it.

For a small operator, the arithmetic is unforgiving. A ventilation plan is only worth the air it actually moves. A methane monitor is only worth the calibration and the person who reads it. A task-trained miner is only as good as the last time someone tested whether he knew what to do when the air went bad.

The mine never reopened. Every class taught in Kentucky since has used it as a case study: the size of the mine has nothing to do with the size of the hazard.