Karst
Karst: a geological phenomenon (process) associated with increased solubility of rock formations (primarily carbonate, sulfate, and halogenous) under conditions of active groundwater circulation, expressed through chemical and mechanical transformations of rocks resulting in the formation of underground cavities, surface sinkholes, depressions, and subsidence (karst deformations)
Source: "SAFETY IN EMERGENCY SITUATIONS. MONITORING AND FORECASTING OF DANGEROUS GEOLOGICAL PHENOMENA AND PROCESSES. GENERAL REQUIREMENTS. GOST R 22.1.06-99"
(approved by the Decree of Gosstandart Russia dated 24.05.1999 No. 177)