Hypogean speleogenesis is the main cave formation process in the Frasassi area. The carbon flux represents an important proxy for the evalution of the different speleogenetic processes. The main sources of CO2 in the underground karst system are related to endogenic fluid emissions due to crustal regional degassing. Another important CO2 source is hydrogen sulfide oxidation. A small amount of CO2 is also contributed by visitors to the parts of the cave open to the public.