Hypogenic Versus Epigenic Origin of Deep Underwater Caves Illustrated by the Hranice Abyss (Czech Republic)—The World's Deepest Freshwater Cave

Extremely deep freshwater filled cave systems are common in karst systems globally. The origin and evolution of such caves are usually attributed to hypogenic (bottom‐up) processes, in which acidic groundwater dissolves limestone from below. However, these deep cave systems can form by epigenic (top‐down) processes, with meteoric waters descending from the surface underground. The Hranice Abyss (Czech Republic), with a reached depth of 473.5 m, is the deepest mapped extent of such a system globally, although its maximum depth is unknown. Multiple geophysical data sets (gravity, electrical resistivity tomography, audiomagnetotellurics, and seismic refraction and reflection) are used to investigate the extent and formation of the Hranice Abyss. The geophysical results suggest the Hranice Abyss extends to depths of ~1 km. Further, we identify structures within the karst, including buried cockpit karst towers with several NW‐SE‐oriented valleys. The new geophysical results from the Hranice Abyss, considered in combination with geological constraints of the region (tectonic evolution and morphology of karst structure), suggest an epigenic formation process, rather than the traditionally invoked hypogenic origin. Formation by epigenic rather than hypogenic processes has implications for local and regional karst history associated with areas hosting deep karst systems. Key Points Hypogenic and epigenic evolution is considered for deep underwater caves Geophysical models show geometry and extent of the Hranice Abyss A new formation model of the Hranice Abyss based on epigenic formation is presented