And one fertile ground for coal-mining ventures is Utah, where the industry has upgraded mechanization in the past 20 years. The state's richest seam of coal swings in the shape of a canted L from Colorado westward, under the Wasatch Plateau, then dropping down south into Emery County — the area in which the Crandall Canyon Mine is situated.

"Utah coal fuels 90 percent of electricity in Utah," says Alan Isaacson, research analyst at the University of Utah.

While it's not clear what caused the Crandall Canyon Mine accident — and the investigation to follow may take some time to complete — it's now known that three of the six miners trapped deep within the mountains are Mexican nationals.

As they have for much of U.S. history, immigrants provide a significant proportion of the labor necessary to operate the mines. Like the boom and bust cycles of the industry, their numbers have waxed and waned over the years.

Mexican miners began arriving in the state in 1910, during their country's revolution, says Utah state historian Philip Notorianni. Immigration slowed down during the 1920s and 1930s, both because of the Depression and immigration restriction laws, he says.

But as Americans left to fight in World War II, immigrants were again brought in from Mexico and Puerto Rico to staff the mines, he says. The 2000 Census provides the latest official indication of numbers of Hispanics working in Utah's coal mines, says Pam Perlich, a senior research economist at the University of Utah. For that year, 8,150 people were employed in the coal mining industry. Of those 596, or 7.3 percent of the civilian labor force, were Hispanic.

"Hispanics didn't really start coming back (to work in the mines) until the 1980s," says Nancy Taniguchi, history professor at California State University Stanislaus and author of "Necessary Fraud: Progressive Reform and Utah Coal."