On the example of Aachtopf (South Germany) it is shown how hypotheses for the occurrence of karst cavities are created. The theses initially contain only abstract ideas. Further observation, especially the impact of particular events, may confirm, discard or further develop the theses. The example presented here refers to a hitherto undiscovered lateral passage leading from the deepphreatic zone into the shallow phreatic or vadose zone. The distance of the assumed branch to the measuring point should be estimated. The basis for the thesis are data on the water level and the water temperature.