What Are Mines?

There are hard-rock underground mines, and there are soft-rock underground mines. Coal deposits, for instance, live in relatively soft sedimentary rock. Gold deposits live in igneous or metamorphic rock, which is relatively hard, as do many other precious metals and valuable minerals including diamonds, copper, silver, nickel and zinc [source: Great Mining].

Even within the hard-rock category, design and extraction methods vary, but almost all revolve around a few basic techniques:

Room and Pillar – For relatively flat ore deposits, with little change in elevation throughout, miners drill an access ramp to the deposit and remove ore in a pattern of holes (rooms) and roof supports (pillars). The rooms can be mined out using conventional charge-and-blast techniques or, more commonly now, with a machine called a continuous miner.

The continuous miner drills into the rock until it forms an excavated room, perhaps 20 to 30 feet (6 to 9 meters), leaving in place a rock pillar to support the "roof" [source: United Mine Workers of America]. The machine moves through the ore, creating rooms and pillars, until the full deposit is covered. A final pass drills through the pillars to recover the ore there, allowing the roofs to collapse behind the machine as it leaves each room.

Cut and Fill – For relatively narrow ore deposits, miners drill an access ramp adjacent to the ore deposit, from the surface down to the lowest point of the deposit. An operator then drives a drill through the ore, creating a drift, or a horizontal cut, from one side of the deposit to the other.

In the hardest rock, no roof-support is needed; in softer rock, bolts may be placed in the roof as the drill progresses. Once the drift is complete, backfill, or waste material, is spread into the open drift, creating a platform for the next pass. The drill drives on top of this backfill to cut another drift through the ore. This continues until the drill cuts a drift across the top of the ore deposit.

This method can be used in wider deposits, as well, by drilling two adjacent access ramps and cutting two adjacent drifts, often called drift and fill.

Cut and fill is for hard rock, as it doesn't feature the support mechanisms inherent in and central to a method like room and pillar. The room-and-pillar approach, on the other hand, crosses easily into the softer stuff – and most coal mines.

Coal, salt, uranium, phosphate and oil shale live in soft rock and there are two primary soft-rock mining methods: longwall and room and pillar [source: Great Mining]. Most coal mining is dine using the room-and-pillar approach described previously [source: United Mine Workers of America]. However, longwall mining is exploding in popularity [source: Great Mining].

Longwall mining is extraordinarily efficient. Rather than drilling through the ore deposit, a longwall machine cuts across it, shaving off slices up to 600 feet (182 meters) long. Those slices drop directly onto a continuously moving conveyor, which carries it to a haulage shaft that lifts it out of the mine.

In longwall mining, the roof supports are built into the machine, sitting between the top of the longwall miner and the roof of the room. As the machine progresses into the ore, the supports move with it, allowing the area behind it to collapse and fill in the excavated area.

The longwall method can recover up to 90 percent of the available ore. The room-and-pillar approach typically recovers about 50 percent [source: Illinois Coal Association].

When the ore deposit in relatively narrow, shorter cuts are made. This variation is called shortwall mining.

The old-school technique of blast mining, that uses explosives like TNT to break up ore, is still in use, but just barely – less than 5 percent of U.S. production [source: Great Mining].

Underground mining, then, is becoming increasingly efficient. It requires less human labor, which puts fewer miners in danger as they work deep beneath the surface. Still, underground mines are not the safest places to be.

The environmental toll of underground mining is significant. It includes air pollution, changes in water-flow patterns, chemical and gas seepage into water supplies and soil, inaccessible fires in abandoned mines, and dramatic changes in land composition that can make the area unusable after the mining operation is done [source: Saxena].

Then there is the human toll. Most mining accidents gain little media attention, especially those involving few casualties or taking place in developing nations. In 2010, almost 2,500 Chinese miners died on the job, none of those attributed to "major accidents" [source: Yang].

That year was a terrible one for mining operations in general. In the United States, a mining catastrophe in West Virginia left 29 dead, the same number that died in an accident in New Zealand. In Chile, 33 miners were rescued in the dramatic incident recounted earlier, but another 45 died in other accidents that same year.

Many accidents occur when the mine props collapse due to earth tremors. Explosions, too, trigger casualties when ventilation systems fail to effectively remove exhaust from mining equipment, coal dust and natural underground gas leaks. Blasting can ignite those gases, leading to deaths from both the explosions themselves and the subsequent collapse of mine structures; a methane-gas explosion killed those 29 miners in West Virginia.

Long-term health problems are a serious job risk, as well. Continually breathing in mineral dust can cause lung diseases like pneumoconiosis or the dreaded black lung. Breathing in welding fumes, radon or mercury (often found in mines) also causes respiratory diseases. Hearing loss from noisy equipment and back injuries from lifting heavy loads are also common [source: Live Science].

Most countries now have laws and regulations designed to address safety and environmental issues. Some require mining companies to return the mined area close to its original state. Others insist that the mining company oversee inspections regularly to ensure they are safe.

And new mining techniques have also decreased the death toll. In the U.S., the mining industry saw thousands of deaths from accidents each year in the early 1900s. This dropped to about a hundred per year in the 1990s, and just 35 in 2012 [source: Mine Safety and Health Administration]. China had 7,000 mining deaths in 2002 but 2,500 in 2010.

While safety has definitely increased in developed countries, it still has a long way to go in some developing countries. What we do know is that valuable mineral deposits won't go to waste. Here's hoping we see a day when underground mines aren't some of the most frightening factories on Earth. Anywhere.