​The Critical Zone
(…) is the “heterogeneous, near surface environment in which complex interactions involving rock, soil, water, air, and living organisms regulate the natural habitat and determine the availability of life-sustaining resources” (NRC, 2001).

In short you could also say "life happens in the Critical Zone".

Find more information here:
https://www.youtube.com/watch?v=aezWKCQxR5U
https://www.youtube.com/watch?v=iv6j7OWXPIE
and 
http://criticalzone.org/national/

Art analogy for the Critical Zone

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