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Temperature is the biggest one. The temperature of the earth's crust increases downward at about 30°C for every kilometer of increased depth. That means a 3 kilometer deep mine might reach temnperatures of nearly 100 C (nearly the boiling point of water. Human body temperature is 37°C and so once you are deeper than a kilometer or so, the air in the tunnels must be cooled. The air cooling systems needed for deep mines are immense and very costly,