The air in the Thermal Gallery (22 000 m 3 has a mean radon content of about 100 kBq/L, temperatures of up to 41 °C, and relative humidities of up to 99%. Due to these unusual conditions and to the first observations of health effects (curing of colds and rheumatic diseases) on mine workers during World War II, the idea was born in 1946 to use that abandoned mine as a big radon inhalation room for therapeutical purposes. This paper describes the historical development of the vast and detailed investigations, the facilities available at that time, of the physical mine properties, and the biophysical and basic medical effects after inhalation of the mine's atmosphere. The physical investigations included the variation of temperature and humidity with artificial and natural ventilation and numerous point measurements of the atmospheric radon content, which continously changed from site to site and day to day. These measurements showed that randon is supplied from the fissures in the rocks in response to changes in barometric pressure. Furthermore, the relation of the amounts of radon to its decay products were determined for the various sites. The biophysical investigations covered the solubility of radon in blood at high body temperatures (hyperthermia) and the determination of the dose in the various tissues and organs in animals and humans after inhalation of radon and its daughters. Individual assessments of personal doses according to the dose and dose distribution studies enabled a detailed investigation of the nonlinear dose dependence of radiation-induced structural chromosome aberrations in the peripheral blood lymphocytes of workers entering the mine and of persons living and working in the spa house in front of the mine and in the area of Badgastein. The basic medical information was obtained from voluntary collaborators and patients, brought into the mine for experimental purposes. They included hematological and cardiovascular tests, observations of mobility enhancements, and personally recorded pain reliefs. Due to these studies, the commercial therapeutic use could be started in 1952. Today, more than 500 patients per day can be treated.