At the southwestern end of Lake Balaton, in the western valley of the north-south oriented hill ridge, is the already known Hévíz Thermal Lake, which dates back to Roman times. The area surrounding the Hévíz Thermal Lake is characterized by a strongly faulted mountain structure. The source of the lake is most likely located along fault lines running north-south and northwest-southeast. Along these fault lines, the Triassic layers have subsided step by step. The source funnel, which is more than 38 meters deep, is located on a Pannonian clay and sandstone layer, while the section above 38 meters lies on a Triassic main dolomite.
The origin of the crater may have occurred when hot water rising along the aforementioned fault lines penetrated the clay and sandstone layers beneath the peat layer, mixed with cooler karst water, and eroded a source funnel with a diameter of approximately 70 x 90 meters in the loose rocks.
The origin of the crater may have occurred when hot water rising along the aforementioned fault lines penetrated the clay and sandstone layers beneath the peat layer, mixed with cooler karst water, and eroded a source funnel with a diameter of approximately 70 x 90 meters in the loose rocks.
The water of the lake flows into the lakebed through the opening of the source corridor. After traveling 3.5 meters into the source corridor, we reach the marvelous spherical chamber of the source cave at a depth of 41 meters. The diameter of the spherical chamber is about 16 meters, and in the middle, a mud saddle divides it into two halves.
To the left of the source corridor is the so-called "cold side," named after the 17.2°C karst spring that was operational at the time of the cave's discovery. To the right of the source corridor is the so-called "warm side," where the waters from 10 different source cracks, with temperatures ranging from 20.2°C to 39.8°C, mix, feeding the lake with 37.8°C water.
Based on the calculation of the geothermal gradient, the depth of the water's origin is estimated to be 900 meters. According to radiocarbon dating (C14), the age of the mixed water feeding the lake is 5600 years (2005 measurement data).
The Discovery of the Spring Cave
The bathhouse buildings were constructed between 1864 and 1869 under the leadership of the Keszthely architect Antal Hencz. During the depth measurements before the construction of the bathhouse, the shape of the lake's bed was revealed. In August 1907, Károly Jordán conducted new measurements at hundreds of points of the lake's bed. He took samples with a sampling device, but during one of the attempts, he became trapped under the steep slope of the lakebed, and the rope broke during the retrieval. At that time, he realized that there might be a horizontal cavity at the bottom of the steep lakebed and that the water might flow into the lake through it. In the 1980s, divers found the depth measurement device during the cleaning of the source opening.
The bathhouse operators were still concerned with the origin of the lake's water, so they contacted the Royal Hungarian Navy Authority in Fiume to send divers and diving equipment for investigation.
On January 25, 1908, a heavy diver sent at the request of Lóczy Lajos could not dive deeper than 22 meters. The attempt was repeated 10 months later, on November 8, 1908, and this time they managed to descend to depths of 12 to 18 meters. It was determined that at a depth of 12 meters, a strong water flow was felt coming from the cracks in the walls. This later proven incorrect conclusion led to the closure of the investigation.
However, the lake kept the secret of its source for a long time. In the spring of 1953, the National Balneological Research Institute, under the direction of hydrogeologist József Cziráki, organized investigations into the Hévíz Thermal Lake and its crater. Divers from the Road Bridge Maintenance Department participated in the underwater investigations. During the dives, they reached 38 meters on several occasions but found no source at the bottom of the crater.
On December 14, 1958, three sport divers, under the leadership of László Wunder, discovered the source of the water at a depth of 38 meters, at the base of the downward bend formed by the continuation of the vertical cliff wall, during a non-exploratory dive. In 1971, at the request of the West Transdanubian Water Directorate, a diver group from the OVH Flood and Drainage Protection Central Organization was tasked with installing a water flow measuring device.
The work was carried out from February 28 to May 31, 1972, under the leadership of diving master György Kovács and István Plózer. The water flow was blocked by wood, stone debris, and cultural waste. After the debris was removed, a new mud layer slid into its place, but on the left side of the water flow, a distinct cross-sectional profile was revealed.
On March 16, 1972, divers helped each other to squeeze into the expanded source opening and were surprised to find that the water was flowing out of a nearly horizontal cave passage. When the cave was illuminated with lamps, the light did not reflect back from anywhere because the large volume of water absorbed the light, suggesting that the cave continued further.
Due to repeated mudslides, experts classified the further exploration as dangerous and halted the investigations.
On the commission of VITUKI, under the professional leadership of Dr. Tivadar Böcker and with the cooperation of AMPHORA KSC, the systematic, scientific exploration of the source system began in 1975. In September 1975, divers, including István Plózer, Sándor Ember, György Kovács, and Károly Szilágyi, were the first to enter the source chamber.
After the significant water extraction from the bauxite mines in the Bakony region, complex temperature protection works were started to monitor the decrease in water flow. After the closure of the bauxite mine in 1991, the water flow began to increase, and by the turn of the millennium, it had improved by about 30%.
With full consideration of the lake's exceptional environmental significance and its classification as a world-renowned thermal bath, only diving research expeditions that monitor and protect the lake's ecosystem are allowed.
