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Engineers have discovered ways to carve through mountains, burrow underneath oceans, and use the ground beneath great cities, all in the name of transportation. But tunnels are not only used for transportation--roads, railways, and subways--but also for mining to remove dirt and rock from the earth to reach valuable resources such as gold and silver.

Modern tunneling and mining was born in 1849 with the invention of the rock drill. Along with dynamite, the rock drill made it possible to excavate large amount of earth, in a tunneling process called drilling and blasting.

Shield tunneling is used to tunnel through soft ground. A vertical shaft is dug to the desired depth, then a sharp tube is pushed through the loose ground. (Try this at home with a mound of damp sand and a paper towel tube.)

In a cut and cover tunnel, a trench is dug then a tube is placed in the trench, and a top is placed over it to create a tunnel. This method is fast and cheap.

In the 1970s, a tunnel-boring machine was invented which can dig a huge hole through solid rock at a rate of twelve feet an hour. The front of the machine has cutter heads that chew into rock; the dirt and rock then go through the intestines of the machine and are put in a collecting bin. While the tunnel-boring machine dramatically cuts the number of workers, its large size and cost make it impractical for most tunnel-digging operations.

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