Boys play basketball in the facilities of Belarus' Republican Clinic of Speleotherapy within a salt mine, as part of their treatment, near the town of Soligorsk, south of Minsk, February 19, 2015. According to the state clinic, more than 7,000 children and adults seek medical treatment for respiratory illness each year in the subsurface chambers of its facilities, located 420 metres underground between layers of potassium and stone salts in an operational salt mine. (Photo by Vasily Fedosenko/Reuters)

Children with doctors of Belarus' Republican Clinic of Speleotherapy descend into a salt mine near the town of Soligorsk, south of Minsk, February 19, 2015. (Photo by Vasily Fedosenko/Reuters)

A girl reads a book while resting in the facilities of Belarus' Republican Clinic of Speleotherapy within a salt mine near the town of Soligorsk, south of Minsk, February 19, 2015. (Photo by Vasily Fedosenko/Reuters)

A teacher exercises in the facilities of Belarus' Republican Clinic of Speleotherapy within a salt mine near the town of Soligorsk, south of Minsk, February 19, 2015. (Photo by Vasily Fedosenko/Reuters)

A nurse of Belarus' Republican Clinic of Speleotherapy speaks on the phone in a salt mine near the town of Soligorsk, south of Minsk, February 19, 2015. (Photo by Vasily Fedosenko/Reuters)

Children play in the facilities of Belarus' Republican Clinic of Speleotherapy within a salt mine, as part of their treatment, near the town of Soligorsk, south of Minsk, February 19, 2015. (Photo by Vasily Fedosenko/Reuters)

Children play billiards in the facilities of Belarus' Republican Clinic of Speleotherapy within a salt mine, as part of their treatment, near the town of Soligorsk, south of Minsk, February 19, 2015. (Photo by Vasily Fedosenko/Reuters)

Workers change a lamp in the facilities of Belarus' Republican Clinic of Speleotherapy within a salt mine near the town of Soligorsk, south of Minsk, February 19, 2015. (Photo by Vasily Fedosenko/Reuters)

Youths chat among themselves as workers walk past in the facilities of Belarus' Republican Clinic of Speleotherapy within a salt mine near the town of Soligorsk, south of Minsk, February 19, 2015. (Photo by Vasily Fedosenko/Reuters)

Youths spend time in the facilities of Belarus' Republican Clinic of Speleotherapy within a salt mine, as part of their treatment, near the town of Soligorsk, south of Minsk, February 19, 2015. (Photo by Vasily Fedosenko/Reuters)

A child looks on upon returning with others after spending time at the facilities of Belarus' Republican Clinic of Speleotherapy within a salt mine, as part of their treatment, near the town of Soligorsk, south of Minsk, February 19, 2015. (Photo by Vasily Fedosenko/Reuters)

Children play table tennis in the facilities of Belarus' Republican Clinic of Speleotherapy within a salt mine, as part of their treatment, near the town of Soligorsk, south of Minsk, February 19, 2015. (Photo by Vasily Fedosenko/Reuters)

Children rest in the facilities of Belarus' Republican Clinic of Speleotherapy within a salt mine, as part of their treatment, near the town of Soligorsk, south of Minsk, February 19, 2015. (Photo by Vasily Fedosenko/Reuters)

Children rest in the facilities of Belarus' Republican Clinic of Speleotherapy within a salt mine, as part of their treatment, near the town of Soligorsk, south of Minsk, February 19, 2015. (Photo by Vasily Fedosenko/Reuters)

Children walk in the facilities of Belarus' Republican Clinic of Speleotherapy within a salt mine near the town of Soligorsk, south of Minsk, February 19, 2015. (Photo by Vasily Fedosenko/Reuters)

Children and their teachers wait before descending into the facilities of Belarus' Republican Clinic of Speleotherapy situated within a salt mine near the town of Soligorsk, south of Minsk, February 19, 2015. (Photo by Vasily Fedosenko/Reuters)

Children walk in the facilities of Belarus' Republican Clinic of Speleotherapy within a salt mine near the town of Soligorsk, south of Minsk, February 19, 2015. (Photo by Vasily Fedosenko/Reuters)

A boy exercises in the facilities of Belarus' Republican Clinic of Speleotherapy within a salt mine, as part of his treatment, near the town of Soligorsk, south of Minsk, February 19, 2015. (Photo by Vasily Fedosenko/Reuters)