Gregory Archombault, age 9, of Coventry, Rhode Island, for his question:
How do they dig salt mines?
Most underground mines are spooky tunnels, grim and gloomy. But underground salt mines are different. The walls of a coal mine are blacker than midnight. But an underground salt mine has pale walls that may glisten like frost. Its tunnels may be as wide as streets and as high as tall churches.
One of the most famous salt mines is in Poland. Its elevator shafts go down, down to 1,000 feet below the surface. On the way, they go through 65 levels of tunnels, called galleries. The different levels have railroads to carry the miners to work. They also carry out carloads and more carloads of rock salt. This lumpy stuff is called "halite." It is the ore from which we get ordinary salt. Halite is a pale rock, glassy brittle and sometimes glassy clear. As a rule, it is tinged with dingy greys, yellows or rusty browns. The people of Poland have been digging their big mine for more than 100 years. In places, the miners have carved statues from the halite, and tourists come from afar to see them and to visit the glistening, wide white tunnels.
Thick layers of halite are found buried deep underground in many part of the world. To get the ore up to the surface, shafts must be dug straight down. The shaft tunnels have elevators. The first level of tunnels are dug at the top of the halite layer, leaving a sturdy roof on top of the mine so that it never caves in. The miners use drills that crack and chip away chunks of brittle rocky ore. Halite is not a very hard rock and, as a rule, a drill is run with a forceful jet of compressed air.
The halite hacked away to make a tunnel is sent up on the elevator. As more ore is dug out, the tunnels grow wide and higher. Expert mining engineers measurethe digging and decide how wide and how high the tunnels should be. They also de¬cide whether the walls need extra beams and supports to keep them safe. The tunnels criss cross each other like streets. They have rails to run the trains of little cars that carry the miners to the day's work and tote the ore along to the elevator shaft. When the mining engineers decide that one level is finished, it is time to start another one. The elevator shaft goes deeper and a thick layer of rock is left between the two levels.
Miners digging small new tunnels need to carry their own beams. The bigger tunnels may be fitted with electric lighting. They may look like wide subways with clean, glistening walls and high ceilings propped up on sturdy, columns and arches. We have salt mines of this sort in Michigan and several other states. The one near Detroit dips down to a level of 1,000 feet. And every year more than a million tons of halite are toted up from its tunnels.
The brittle halite rock must be smashed and bashed into small pieces. The underground miners use electric crushers to break up the big chunks for loading the 1 ittle cars. At the top of the elevator stands a tower, maybe as tall as an eight story building. This is the tippler. The ore from the mine is sent to the very top. As it comes down the lumps and chunks are crushed into crystals and sifted through screens. Most of it will be used in industry. Of every 100 pounds, less than five pounds will be refined and purified into salt for human food.