It wasn't all that many decades ago when coal mining was largely a back-breaking manual job, as in the 1950s song 16 Tons by Tennessee Ernie Ford.

Some people say a man is made outta' mud
A poor man's made outta' muscle and blood
Muscle and blood and skin and bones
A mind that's a-weak and a back that's strong

You load sixteen tons, what do ya get?
Another day older and deeper in debt
Saint Peter don't you call me 'cause I can't go
I owe my soul to the company store

Today, a small number of miners using machines can do the work of hundreds of manual labor miners. Some mining machines aret developed to the point where they can be operated remotely but not every mine has them. Remotely control machines can have problems of their own. For example, I heard earlier today that the robot they sent into the mine to try and find the trapped miners got stuck in the mud.

From the Department of Labor:

During the 1990s, production of both minerals and coal increased. Given the more volatile price of metal, its production fluctuated more than that of nonmetallics. However, employment in both sectors declined significantly as new technology and more sophisticated mining techniques increased productivity, allowing growth in output while employing fewer workers. Most mining machines and control rooms are now automatic or computer-controlled, requiring fewer, if any, human operators. Many mines also operate with other sophisticated technology such as lasers and robotics, which further decrease the number of workers needed to mine materials.

From the same report (emphasis added):

Working conditions in mines and quarries can be unusual and sometimes dangerous. Underground mines are damp and dark, and some can be very hot and noisy. At times, several inches of water may cover tunnel floors. Although underground mines have electric lights, only the lights on miners’ caps illuminate many areas. Workers in mines with very low roofs may have to work on their hands and knees, backs, or stomachs, in confined spaces. In underground mining operations, dangers include the possibility of an explosion or cave-in, electric shock, or exposure to harmful gases.

Workers in surface mines and quarries are subject to rugged outdoor work in all kinds of weather and climates. Some surface mines shut down in the winter because snow and ice covering the minesite makes work too difficult. Physical strength and stamina are necessary because the work involves lifting, stooping, and climbing. Surface mining, however, usually is less hazardous than underground mining.

In 2003, the rate of work-related injury and illness was 3.7 per 100 full-time workers in metal mining, 3.7 in nonmetallic minerals, and 6.2 in coal mining, compared with 5.0 for the entire private sector. Mining illnesses and injuries have steadily declined over the years because of stricter safety laws and improvements in mining machinery and practices. Although mine health and safety conditions have improved dramatically, dust generated by drilling in mines still places miners at risk of developing either of two serious lung diseases: Pneumoconiosis, also called “black lung disease,” from coal dust, or silicosis from rock dust. The Federal Coal Mine Health and Safety Act of 1969 regulates dust concentrations in coal mines, and respirable dust levels are closely monitored. Dust concentrations in mines have declined as a result. Underground miners have the option to have their lungs x-rayed when starting a job, with a mandatory follow-up x-ray 3 years later, in order to monitor any development of respiratory illness. Additional x-rays are given every 5 years, on a voluntary basis. Workers who develop black lung disease or silicosis may be eligible for Federal aid.