The basic problem of maintaining a civilization, according to Tainter, is maintaining a positive energy balance; spending less energy than it consumes. With a negative energy balance, gradually the entire population starves to death. For this reason, all historical human populations have lived on the surface of the Earth, not inside it, because on the surface there is lots of readily available sun energy, and even inefficient ways of harvesting it (such as plants) suffice to support humans. Also, the conditions are relatively mild. The worst problem is that parts of the surface do not have salt water covering them, which is a problem we learned to cope with some hundreds of millions of years ago, with means duch as lungs, eggs, and amniotic sacs. Inside the Earth, all of our necessities must be provided by clever contrivances; not just food, but light, oxygen, low pressure, cool, dry (we have become accustomed to it in the last few hundred million years) , and sanitation.
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