FRANKFORT, Ky. -- The number of miners killed on the job in the United States fell for a second consecutive year -- to 34, the fewest since officials began keeping records nearly a century ago.That was down from the previous low of 52 in 2008.U.S. Mine Safety and Health Administration documents show 18 of the deaths occurred in coal mines, down from 29 in 2008; and 16 were in gold, copper and other types of mines, down from 22 in 2008. Most involved aboveground truck accidents on mine property, though some of the deaths resulted from rock falls and being struck by machinery.Obama administration mine safety czar Joe Main said the numbers are encouraging, but he won't be satisfied until no miners are killed on the job."I think that's accomplishable, if you look at where we came from, and where we've come to," Main said.The latest statistics are vastly improved, he said, from a century ago when hundreds, sometimes thousands of miners were killed each year.The deadliest year in recorded U.S. coal mining history was 1907, when 3,242 deaths were reported. That year, the nation's most deadly mine explosion killed at least 358 people near Monongah, W.Va.Main credits the decrease in deaths over the past year to beefed-up enforcement and stricter regulations in the wake of a series of mining disasters over the past four years in Kentucky, Utah and West Virginia.In 2006, 73 miners were killed, including 12 who died in a methane explosion at the Sago Mine in West Virginia and five who died in a similar explosion at the Darby Mine in Kentucky. In 2007, 67 miners died, including six who were killed in the collapse of the Crandall Canyon mine in Utah.Coal states reacted by revamping their mine safety laws, and Congress toughened federal rules that brought a variety of advances. Among the improvements are caches of oxygen stashed in underground mines in case miners are trapped, refuge chambers to provide shelter in emergencies, and a communications system to allow underground miners to talk with colleagues on the surface.Steve Earle, United Mine Workers of America international vice president for the Midwest, said while those were important improvements, getting inspectors into the field is the key."I can say without reservation that the safest day coal miners have is when inspectors are in the mines," he said. "The more we can put our inspectors in the mines, the safer those mines will become and the closer we will come to zero fatalities."Mine safety advocate Tony Oppegard, who has successfully lobbied to triple the number of mine inspections conducted in Kentucky, said mining remains a dangerous occupation."Everyone who's involved in mine safety has to be extremely vigilant," he said. "There's very small margin for error in coal mining. The smallest mistake can cost a miner his life."Kentucky led the nation in mining deaths last year with six in coal mines and one in a limestone quarry. That was followed by West Virginia and Alabama, each of which had three coal miners killed.Illinois, Louisiana, Missouri, Pennsylvania, Tennessee and Texas each had two miners killed in coal, salt, alumina, zinc or sand and gravel operations. Arizona, Arkansas, California, Georgia, Iowa, Nevada, Ohio, Virginia and Puerto Rico each had one miner killed in either clay, copper, gold, lime or sand and gravel operations."It's never positive when you have numbers like that, but it could have been worse," said David Moss, spokesman for the Kentucky Coal Association. "We're always striving for that goal of zero. That's what we work toward every single day."Main credited cooperation between regulatory agencies, coal companies and miners with making mines safer, which led the decrease in workplace deaths."It is historic," he said. "And it does tell us we can achieve a point in time when we have no fatalities."