Layers of rock overhead rumble like thunder, dirt and pebbles rain down. What is a miner to do?

"You run for your life," said Tim Miller, who toiled in Kentucky's mines for more than two decades.

People who work underground in coal mines know what it is like to scurry through darkness to evade falling rocks. Miners say that is part of the job, especially when it comes to digging coal from the very pillars that keep layers of rock from collapsing in on them.

Although it may seem strange, generations of miners have been cutting away those pillars to increase coal production in a practice known as retreat mining. It is legal and considered standard procedure. But it has claimed the lives of 17 coal miners in the past seven years.

In Kentucky alone in the past 14 months, four miners have been crushed in rock falls during retreat mining.

"You're definitely playing Russian roulette," said Miller, now an organizer for the United Mine Workers of America, which spells out in its contract that members can withdraw from any section of mine they believe is unsafe. "You remove those pillars, the roof is coming down. It's inevitable."

Retreat mining is almost exclusively a southern Appalachian practice, with 83 percent of the nation's retreat mining operations in eastern Kentucky, southwestern Virginia and southern West Virginia. Those three states account for the most retreat mining because they also account for most of the nation's underground coal mining activity. They also account for the most deaths -- 14 of the 17.

Kentucky Gov. Ernie Fletcher (R) has commissioned a study to look for ways to make retreat mining safer, an initiative he announced soon after two miners were killed in a rock fall Aug. 3 in a mine near Cumberland.

Investigators said the mine roof gave way without warning in the Stillhouse Mining operation, burying the miners under an 11-foot layer of rocks. The body of Brandon Wilder, 23, was found within hours, but it took searchers more than three days to recover the body of Russell Cole, 39.

About half of Kentucky's underground coal mines do retreat mining, said Bill Caylor, president of the Kentucky Coal Association.

Caylor said he believes retreat mining, also known as pillaring or secondary mining, is important because it allows companies to get more coal.

Underground, coal miners first do advance mining, digging into the mountain to remove coal that now sells for $50 to $60 a ton. In that process, 30 to 40 percent of the coal is removed by cutting a maze of 20-foot-wide tunnels through it, said J. Steven Gardner, a Kentucky mining engineer. The mining leaves coal pillars 25 to 100 feet across and a network of tunnels with coal pillars in between.

When companies have advanced as far as possible, they begin retreat mining. In retreat mining, the companies usually remove an additional 20 percent of the coal from the pillars, though that percentage can be much higher if geology permits.

Terry Hock, chief of the roof control division of the U.S. Mine Safety and Health Administration, said coal companies might partially remove the pillars by mining through the middle of them. In some cases, they remove the pillars, leaving the mine ceiling unsupported.

State and federal regulatory agencies review every company's plan for retreat mining for safety and workability. "They're scrutinized heavily," said Paris Charles, head of Kentucky's Office of Mine Safety and Licensing.

Mine Safety and Health Administration deputy administrator John Langton said most of the miners killed during retreat mining were setting up posts or jacks to help hold up the mine ceiling or were operating the machines that chew the coal loose.

For now, UMW President Cecil Roberts said the union has yet to take a position against retreat mining but supports Kentucky studying a practice he compared to "going under your house and taking the support beams out."

At the Kentucky Coal Museum, Jennifer McDaniels shows a model of mine pillars, foreground, and the effects of retreat mining.