|MadSci Network: Earth Sciences|
Tammy, In researching the air issue, I didn't find any reference to the depth at which air could or couldn't travel. Many references to "bad air" dealt with the accumulation of methane and carbon di/monoxide. The air quality issue also dealt with the accumulation of coal dust. Therefore, ventilation was used primarily for the removal of the bad air contributors. Ventilation was accomplished through drafting or through the use of ventilation shafts. Mechanical fans and cylinder air were mentioned when air flows couldn't be reached by the previous methods. However, these were costly when compared to gravity fed systems. At 5km the temperature of the rock can reach 700 degrees Celsius. The temperature of a certain depth is dependent on what type of rock your mining into. Another limit to how deep we can mine is the weight of rock. At 3.5km the pressure of rock laying above you is 9,500 tonnes per meter squared. When rock is removed through mining this pressure can rise threefold in the surrounding rock. This, and the cooling of the rock causes a phenomenon called rock bursts. Have a good day and stay away from any old mines!
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