The frontier beneath our feet

Following the simple question as to where water goes when it rains leads to one of the most exciting frontiers in earth science: the critical zone-Earth's dynamic skin. The critical zone extends from the top of the vegetation canopy through the soil and down to fresh bedrock and the bottom of the groundwater. Only recently recognized as a distinct zone, it is challenging to study because it is hard to observe directly, and varies widely across biogeoclimatic regions. Yet new ideas, instruments, and observations are revealing surprising and sometimes paradoxical insights, underscoring the value of field campaigns and long-term observatories. These insights bear directly on some of the most pressing societal problems today: maintaining healthy forests, sustaining streamflow during droughts, and restoring productive terrestrial and aquatic ecosystems. The critical zone is critical because it supports all terrestrial life; it is the nexus where water and carbon is cycled, vegetation (hence food) grows, soil develops, landscapes evolve, and we live. No other frontier is so close to home. Plain Language Summary One of the newest and most exciting frontiers in earth sciences is the critical zone: the thin, dynamic skin of the terrestrial Earth that extends from the top of the vegetation canopy through the soil and down to fresh bedrock and the bottom of groundwater. The critical zone is where water moves, vegetation grows, roots spread, organic matter decomposes, soil develops, and rock weathers. It's also where we, and most life, lives, and is therefore 'critical'' to our survival. Studying the critical zone has rapidly become an international and interdisciplinary science effort utilizing field studies, long term observatories, and new geophysical measurement techniques. These studies are revealing insights into a broad range of previously unexplored topics: where do trees get their water, how does rock weather, and where does water go when it rains. Understanding the critical zone is vital to addressing key environmental and social problems: maintaining soil productivity in intensively managed landscapes, ensuring that forests don't die during droughts, and improving landscape resilience to wildfires, floods, and hurricanes. Today, the term critical zone'' provides an essential organizing principle for the earth and biological sciences just as ecosystem'' did for ecology half a century ago.