For hundreds of years engineers and scientists have tried to find better ways to rescue miners trapped underground by cave-ins.
Coal mining is one of the world's most hazardous occupations.
In China alone, officials acknowledge there were thousands of mining accidents in 2008, resulting in more than 3,000 deaths. China watchers say the official number of mining fatalities is probably a gross underestimate.
The deaths are caused by fires, floods, explosions and collapses in crumbling and, occasionally, illegal mines.
The challenge in a mine cave-in is actually pinpointing the location of the miners before they die of asphyxiation.
Researchers in Utah are developing a system they say could significantly reduce the time it takes to find trapped miners, increasing the likelihood that drilling a rescue shaft will be successful.
University of Utah in Salt Lake City Geology Professor Sherif Hanafy, says the system involves installing metal plates a short distance apart on the walls of mine shafts. He says when one of the metal plates is struck with a hammer it creates a unique "seismic fingerprint", or ground wave. Those ground waves can then be pinpointed with sensitive listening devices called geophones.
In a cave-in, miners would hit the plates, the listening device would locate the origin of the tapping, and rescue workers would know where to dig.
Hanafy says the system is relatively inexpensive and was 100 percent accurate when tested in an old copper and utility mine. But actual cave-in conditions are very different, so the professor says researchers created a computer model to test the system under different scenarios.
"What if there is more than one group trapped inside the mine? And what if one of the group[s] is hitting [the plates] more frequent or stronger than another group," said Hanafy. "And the technique looks like it solves all of them."
The authors of the study say such a system could be built for about $100,000 in a typical mine, a figure they say is relatively inexpensive.
Researchers are trying to develop a similar rescue system in West Virginia, where an explosion in the Sego Mine trapped and killed 12 coal miners in 2006. Rescue workers had tried for days to locate the miners but were unsuccessful.
University of West Virginia Mining Engineering Professor Keith Heasley says researchers have been testing a mobile system that could be assembled within a few hours on top of the site where rescuers think miners are trapped.
Heasley says the system was tested at two active coal mines.
"At 450 feet [137 meters] deep, we were very successful at hearing people pounding on the roof. We went to a location that was 800 to 1,000 feet [244 to 304 meters] deep, and we were very unsuccessful at hearing people pounding on the roof. [There was] A lot of background noise at that second location," he said.
Heasley is optimistic that with better sensing techniques, a successful mine rescuing system will eventually be developed. But he says it will take a lot more work to develop a system to locate trapped coal miners further underground.
The Utah researchers described their mine rescue system in an article published in The Leading Edge, a journal of the Society of Exploration Geophysicists.