In the hydrogeological sense the karst aquifer of Classical Karst / Kras is a uniform unit, but politically divided between two countries. The main part of the aquifer is located in Slovenia, but the whole karst coast and the springs area are located in Italy. To understand its functioning and to preserve it properly a close co-operation between experts from both countries is necessary. Classical Karst / Kras is a limestone plateau of 900 km2 that extends from SE-NW direction between Brkini hills in Slovenia and Isonzo River in Italy. To understand the functioning of the transboundary karst system many researches were performed in a close co-operation between Italian and Slovene researchers. One of the primary goals was the protection of the aquifer, in which large quantities of groundwater are stored. The springs of the Timavo River are one of the highest-discharge regions in the Mediterranean region (medium discharge of 40 m3/s, maximum of 175 m3/s). Close to the springs, on the Slovenian side, groundwater is pumped for the supply of several municipalities. In Italy, the Sardos and Moschenizze Nord springs are still used at present for water supply of Trieste. The hydrodynamics and chemical characteristics of springs are well known, but there is a lack of informations about autogenic and allogenic recharge. Only few data are available about hydrodynamic behavior within the hydro-structure. For these reasons the spatial hypogean development of the karst phenomena is very unpredictable. Karst voids organization is driven by several aspects: geological and structural settings, climate characteristics, geomorphological context etc. Due to the high heterogeneity of the underground karstification is still very complex to model the groundwater circulation, to define the underground karstification development and the karst voids connection especially in a mature karst.