"The most dangerous type of mining ... "

WASHINGTON — The method of mining used at the Utah mine that collapsed Monday, trapping six miners, has a history of being disproportionately deadly, according to federal safety studies.

The Crandall Canyon mine collapse happened while miners were engaged in a method called "retreat mining," in which pillars of coal are used to hold up the roof in an area of the mine. When that area is completely mined, the company pulls the pillars and grabs the useful coal, causing an intentional collapse.

It is "the most dangerous type of mining there is," said Tony Oppegard, a former top federal and Kentucky mine-safety official who is now a private attorney in Lexington, Ky., representing miners.

According to the American Society of Safety Engineers, retreat mining requires precise planning and sequencing to ensure roof stability while the pillars supporting the roof are removed.

The reason the practice is used is that it pays off: The last bit of coal taken from pillars is pure profit, Oppegard said. Plus, if someone violates rules during pillar removal and there is a collapse, the evidence of rule violations is gone, he said.

Retreat pillar mining is one of the biggest causes of mine roof-collapse deaths, according to studies done by the National Institutes of Occupational Safety and Health, which concluded that "a coal miner on a pillar recovery section was more than three times as likely to be fatally injured" in a roof collapse than colleagues in other parts of a mine.