Upon their return from orbit, astronauts have often remarked on their profound sense of the beauty, interconnectedness, and fragility of the planet Earth. From space, human boundaries disappear and the global sweep of natural forces becomes apparent, protected only by the thin blanket of the atmosphere.

Most of the life-sustaining processes on Earth — and, indeed, all living things — exist in a narrow band close to the surface of the planet. Scientists have named this near-surface environment "the Critical Zone." In 2001, the National Research Council defined the Critical Zone as 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."

The Critical Zone includes the lower atmosphere, land surface, vegetation, and water bodies and extends down through layers of soil to the edge of groundwater penetration. The Critical Zone is the most heterogeneous portion of the Earth — it is where many dissimilar components meet and interact. The Critical Zone is characterized by interfaces: the air-water interface, for example, where gases and minerals are exchanged; or the root-soil interface, where microbes facilitate the exchange of nutrients.

An array of important physical, chemical, and biological processes and reactions occurs in the Critical Zone. Some processes are quick, while others take eons. Some processes are widespread, while others are localized. These processes impact the mass and energy exchange necessary for biomass production, chemical recycling, and water storage. They also control the transport and cycling of contaminants. They determine the health of the ecosystem and its inhabitants, including humans.

Getting a better grasp of the complexity of the Critical Zone has become a research priority for many scientists. The Center for Critical Zone Research at the University of Delaware brings together an interdisciplinary team of scientists and engineers who study the interfacial processes that take place in the Critical Zone.

Just like interfaces in the Critical Zone, the interface between different types of scientists is "where the action is" in terms of research. For example, by combining the expertise of a soil scientist, a microbiologist, and a materials engineer, we can devise new methods to study how certain bacteria in the soil react to the presence of a toxic metal such as arsenic. Perhaps this will lead to new ways of remediating contaminated soils, and in the long run, improve human health by reducing arsenic exposure.