Hévíz is a spa town in Zala County, Hungary, about 8 kilometres (5 miles) from Keszthely. The town is located near Lake Hévíz, the world’s second-largest thermal lake, but biologically the biggest active natural lake. Its temperature is affected by the combination of hot and cold spring waters, coming from 38 meters underground.
The water breaks out from a spring cave, at approximately 410 liters per second, with a temperature of 40 °C (104 °F). The biological stability of the lake is shown by the temperature of the water, which has not changed for years and even on the coldest winter days doesn’t drop below 24 °C (75 °F). That makes bathing possible in the lake year round. In the summer, the water temperature can reach 37 °C (99 °F).