Rod Anderson, left, and Mike Light work on drilling rig on hillside in Tallmansville, W.Va, following explosion in mine there

U.S. government health and safety officials are investigating the cause of the recent explosion at a West Virginia coal mine, which killed 12 miners. The accident was apparently an aberration in an industry that has prided itself on miner safety during a time of unprecedented expansion.

The coal mining industry in the United States is booming. Mine companies operate in 27 states, from West Virginia in the east to Montana in the west, producing a total of about one billion tons a year, or more than a third of the world's coal supply. The U.S. economy is dependent on coal production. Coal-fired power plants generate more than 50 percent of the nation's electricity.

More than half the nation's coal is mined underground by thousands of men and women who daily risk injury - and death. But the occupation has become much safer since the late 1960's, when the U.S. Congress passed laws requiring federal mine inspections.

"Mining is an inherently dangerous occupation. Those responsible for inspecting the mines are actively inspecting them. They find things wrong, and make recommendations for improvements," says Lewis Wade, an expert on coal mining at the National Institute for Occupational Safety and Health in Washington, D.C.

Federal investigators are now looking at the possible causes of the West Virginia coal mine disaster, in particular, to see if unventilated gases might have set off the underground explosion.

"In mining what you try and do is move fresh air to constantly dilute the methane gas," Wade explains. "Methane is a potential fuel for an explosion in a mine. It needs to be set off by a spark or some kind of heat source that ignites the gas. Also, in coal mines you have coal dust created by grinding of the coal. There are other things. That coal dust is also potentially explosive."

A trapped miner of Dongfeng Coal Mine is carried out by his fellow workers

The inherent risks of coal mining are grimly evident in China, where nearly 10,000 miners are estimated to have died in underground accidents since the beginning of 2004. Mining accidents are less common in most advanced industrial countries, thanks to improved mine-safety technologies and stricter government oversight. In the United States, the federal agency that inspects coal mines is called the Mine Safety and Health Administration or MSHA, for short. (http://www.msha.gov/) Former MSHA director J. Davitt McAteer says that since 1984 there have been just three fatal explosions in U.S. mines, before this latest one.

"We've made a lot of progress since the 1970's in this country, but what this accident suggests is that we must be vigilant," he says. "We should be able to mine year in and year out without a disaster. This [latest explosion] points to the fact that we need to renew our efforts to really eliminate disasters and bring those numbers down."

McAteer says that in cases of major disasters - like fires and explosions - better communications equipment is required.

"The communication [device currently in use] is wire-based radio from down in the bottom up to the top," he notes. "Once an explosion occurs, of course, that's knocked out. That's the first thing that goes. We need something that's wireless. We need to have something that sends signals and can't be disrupted by explosions. We need something that goes through the earth. There are some techniques being developed, but we haven't spent the energy and time and money to get that to happen."

Davitt McAteer says mining companies should also improve the portable emergency oxygen supply miners take with them underground.

The fact is they aren't adequate to bring people out and they need to be updated. They're 30 years old."

Still, officials say the United States has the safest underground coal mining system in the world, due to laws that require regular inspections; technology, like sensing devices that can detect deadly gases; and giant machinery that can do much of the digging work and put fewer human lives at risk.