This paper outlines the conclusion from an analysis of 18 large cave systems around the world, comprising more than 1500 km of conduits. The 3D geometry of complex cave systems have been analysed in relation to their geological context as well as their hydrogeological boundary conditions. The methodology allowed for the first time statistical evidence of the inception horizon concept. Thereby it confirms that the development of karst conduits under phreatic conditions is strongly related to a restricted number of so called inception horizons. An inception horizon is a part of a rock succession that is particularly susceptible to the development of karst conduits because of physical, lithological or chemical deviation from the predominant carbonate facies within the limestone sequence. It passively or actively favours the localisation of dissolutional voids.