The Earth's Greatest Porous Media

Abstract How deeply does modern meteoric water circulate into the continental crust? How deep is the Earth's Critical Zone (CZ), the top layer of the continental lithosphere that co‐evolves with the atmosphere, hydrosphere, and biosphere, extending from vegetation canopy down to fresh bedrock and the base of active groundwater circulation? The answers depend on the substrate porosity and permeability. The difficulties in seeing the subsurface have prevented a global 3D vision. Site studies reveal that the CZ is far deeper than agricultural soils, commonly exceeding 10s but can reach 100s of meters in karsts or ancient shields in the tropics. We know that it is the main terrestrial water‐storage tank for societies and ecosystems. We know that it is a biogeochemical factory transforming rocks into porous growth media, releasing nutrients for life, mobilizing solutes, and consuming atmospheric CO 2 . Unlike in the atmosphere and the ocean, water on land mostly resides in and moves through the porous earth materials. Thus, we have a porous media problem. Yet we know little about the shape and size of this porous media, hindering its inclusion in Earth‐system models. CZ scientists are in the best position to deliver such a planetary‐scale vision of the upper lithosphere, so that we can fully comprehend the functions of the land in the whole‐Earth system in the past and the future. We have enough knowledge to start.