Why go underground?

For hundreds of thousands of years, our natural domain has been a principally two-dimensional space : the surface of the ground.
Urged by necessity, curiosity, and even by temerity, we have always tried to escape from this space, either by widening it, which is only possible in a very restrictive sense, or by searching to utilize the third dimension, upwards or downwards. In these efforts, we have always encountered great difficulties that have been overcome thanks only to an astonishing tenacity.

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