High in the Austrian Alps, patients wait in bathrobes and sandals to take a small train deep inside a disused mine to breathe in a natural radioactive gas, hoping it will help what ails them.
After a bumpy 20-minute ride, they arrive in one of four stations at the "Heilstollen," or "health mine," in the heart of Radhausberg mountain south of Salzburg -- a brief trip to the "tropics" while never leaving Europe.
Once there, they undress, lie down on camping beds and take a cure that has drawn millions of people worldwide to the unexpected heat, humidity and special atmosphere two kilometers (just over a mile) inside this Alpine peak.
Bad Gastein used to be a gold mine, and produced more than 800 kilos (1,760 pounds) of the precious metal each year during its 16th-century heyday. These days it attracts those hunting for a different type of good fortune, some longed-for relief from rheumatism, arthritis, asthma and skin conditions.
"The secret of its medical success comes from the ideal combination of three factors -- a constant temperature between 37 and 41 degrees C (99 to 106 degrees F), near 100 percent humidity, and a high natural level of radon gas," said Franz Zaver Rieser, a manager at the medical centre based at the opening of the mine.
"We receive 10,000 patients a year, and over 55 years in operation about three million patients have passed through the various mine areas," he said.
The beneficial properties were discovered around 1946, when miners searching unsuccessfully for the last traces of gold noticed their rheumatism, skin problems and respiratory difficulties had temporarily disappeared or were considerably improved after spending time underground.
Following a series of scientific studies on its effects, the mine was reopened in 1952 as what the organisers say is the world's only "health mine."
Gernot Strauss, part of the medical team at the centre, stressed that the mine treatment cannot cure ailments, only alleviate their symptoms. But he said the effect can last up to nine months.
"There is as yet no cure for rheumatism," said Rieser, but "more than 90 percent of patients feel better after treatment."
The vital component, he explained, is a stimulating effect of radon, a naturally occurring radioactive gas element that in high concentrations and long exposures is said to carry serious health risks.
At Bad Gastein, low doses of this colourless and odorless gas have been found to boost the ability of human cells to repair themselves.
The World Health Organisation issues regular warnings that high exposure to radon can increase the risk of lung cancer. It has been a major headache for some home and office owners in radon-prone areas, notably parts of the United States where the US Environmental Protection Agency website says it is "the leading cause of lung cancer among non-smokers ... (and) the second leading cause of lung cancer in America and claims about 20,000 lives annually."
But Bad Gastein organisers say patients are given one day without inhaling radon in between two days of treatment, and say the gas is out of the patients' bodies within three hours.
"The 50-odd mine staff are subject to 20 times more exposure than patients, but we remain below official exposure levels," said Rieser.
Depending on the gravity of the illness, doctors recommend up to 12 trips of one hour each to the mine over a four-week period, a course which could cost up to 600 euros (800 dollars).
Patients come from all over for the treatment, Rieser added, remembering "one 81-year-old woman who came all the way from Australia."