DIRECT HUMAN EXPLORATION OF THE TERRESTRIAL SUBSURFACE

Caves and karst present a unique opportunity to observe terrestrial subsurface processes firsthand, including geochemistry and microbiology. Although humans move with difficulty in karst landscapes belowground, advances in caving and cave diving technology and techniques are opening more and more areas to scientific inquiry. These sites, often first identified by citizen scientists, include redox interfaces that fuel isolated microbial ecosystems. These environments are of great interest both for astrobiology and the early earth, and as model systems for understanding microbial ecology and subsurface biogeochemistry. The past decade of research in sulfidic caves in central Italy has led to new insights into microbe-animal symbioses, microbial niche dimensions, microbial biogeography, and redox processes and microbe-mineral interactions that create conduits for fluid flow in the deep subsurface. These systems developed over the past 2-5 million years at depths up to 600 m below ground surface, involving anoxic waters with sources up to several kilometers deep. Chief advantages of direct human exploration of these anoxic and microoxic subsurface environments include the ability to deploy in situ geochemistry and microbiology techniques, and direct observation of spatial variability and changes due to seasonal and yearly variations in hydrologic conditions. This and other recent examples suggest a bright future for new discoveries enabled by direct human exploration of the terrestrial subsurface through karst windows.