It is a notion invented by Earth scientists to bring together many different disciplines that have not collaborated sufficiently in the past. Whether you study water, soil, plants, rocks, weather, or animal life, all of those phenomena are confined to a very thin domain when compared to the whole of planet Earth, as viewed from outer space. The Critical Zone is just a few kilometers thick. It is the only region of the Earth that has been transformed by life over many eons. It is also the only part of the world that you have any chance to experience directly with your senses. Although human activity is barely visible at the planetary scale–not to mention the scale of the universe–it is hugely disruptive at the scale of this thin, fragile, and highly complex Critical Zone. This is why we need to learn how it behaves just as much as we need to know how our body functions. And yet, although we have a vast number of tools and instruments to monitor our bodily health, we don’t have many to monitor the health of the Critical Zone in which we humans live–as well as all of the life forms on which we depend. This domain is called “critical” because this tiny part of the Earth on which we are totally dependent has entered into a sort of intensive care. All efforts should be made to sustain its well-being.