PRINCETON, Ind. (AP) - An Associated Press reporter recently traveled deep into an underground coal mine in southern Indiana, the Gibson North mine owned by Alliance Resource Partners. Here are five things to know about underground coal mining, which accounts for about a third of U.S. coal production:


Underground coal mines are dangerous places to work, so plenty of safety gear and training is required. Even for a short tour, media members were required to wear steel-toed boots, a hard hat with a light, safety glasses, reflective clothing and a heavy belt with a 10-minute oxygen canister. Before entering the mine, there was a short training session with oxygen breathers and an explanation of mines escape routes in case of an incident.


The walls (called the coal rib), roof and floor of the mine underground are not black like coal, but gray, thanks to a dusting of crushed rock that's applied to help mitigate fires. Coal dust is highly combustible, and in the event of a blast, the rock dust mixes with coal dust and prevents the spread of flames. Underground mines are also required to have ventilation plans that keep surface air moving in and out deep inside the mine, to keep flammable methane gas levels low.


The use of mechanized diggers and haulers in underground mining has increased efficiency but also reduced the number of workers needed underground. The Gibson mine has 350 workers, and many of them are machine operators. One of the continuous mining machines seen during the tour weighs 68 tons and cost nearly $2 million. It has a spinning drum with carbide-tipped teeth that easily cut into the coal seam.


Coal mines like Gibson's that use a "room and pillar" technique to remove the coal have a series of tunnels that surround wide square pillars. From above, it would look like a checkerboard with thicker borders between the square spaces. Continuous mining machines dig the coal, and once they get a certain distance into a tunnel, they back out so bolting machines can go in and fortify the roof against collapse.


The Gibson North mine last year produced 3.8 million tons of coal. All of it is shipped off and used by utilities in the nearby area and southeast U.S. to generate electricity. The north complex at Gibson, which opened in 2000, has about eight years left of mineable coal. The South complex opened last year has a coal supply that will last until 2032.