Hundreds of patients make an annual pilgrimage to the village of Wieliczka in southern Poland, where the medieval salt mines offer one of the more unusual forms of respiratory therapy.

For less than $700—a cost which many health insurers cover—asthmatic or allergic patients can spend 14 days deep in the mine’s 180-mile network of subterranean galleries, breathing the therapeutic air.

"This air is absolutely beneficial for asthmatics, because they don’t have any contact with allergens," says lung specialist Marta Rzepecka. Through a daily regimen of exercise and yoga-style aerobics, patients enjoy the mine’s high levels of humidity and sodium chloride, which help speed the regeneration of the mucous membranes in 90% of participants.

"We also see an improvement in the overall functioning of the respiratory system," says physiotherapist Dorota Wodnicka. “[The patients] have less feeling of asphyxiation. The children take fewer antibiotics and they have fewer symptoms."

The mines not only house the sanatorium but have also become a major tourist attraction and a UNESCO World Heritage site that draws more than a million visitors a year.