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. REUTERS/Vasily Fedosenko (BELARUS - Tags: HEALTH SOCIETY TPX IMAGES OF THE DAY) FOR BEST QUALITY IMAGE ALSO SEE: GF2EB341HXF01 

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. 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. REUTERS/Vasily Fedosenko (BELARUS - Tags: HEALTH SOCIETY) 

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. 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. REUTERS/Vasily Fedosenko (BELARUS - Tags: HEALTH SOCIETY) 

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. 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. REUTERS/Vasily Fedosenko (BELARUS - Tags: HEALTH SOCIETY) 

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. 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. REUTERS/Vasily Fedosenko (BELARUS - Tags: HEALTH SOCIETY) 

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. 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. REUTERS/Vasily Fedosenko (BELARUS - Tags: HEALTH SOCIETY) 

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. 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. REUTERS/Vasily Fedosenko (BELARUS - Tags: HEALTH SOCIETY TPX IMAGES OF THE DAY) 

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. 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. REUTERS/Vasily Fedosenko (BELARUS - Tags: HEALTH SOCIETY) 

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. 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. REUTERS/Vasily Fedosenko (BELARUS - Tags: HEALTH SOCIETY) 

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. 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. REUTERS/Vasily Fedosenko (BELARUS - Tags: HEALTH SOCIETY) 

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. 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. REUTERS/Vasily Fedosenko (BELARUS - Tags: HEALTH SOCIETY) 

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. 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. REUTERS/Vasily Fedosenko (BELARUS - Tags: HEALTH SOCIETY) 

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. 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. REUTERS/Vasily Fedosenko (BELARUS - Tags: HEALTH SOCIETY) 

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. 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. REUTERS/Vasily Fedosenko (BELARUS - Tags: HEALTH SOCIETY) 

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. 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. REUTERS/Vasily Fedosenko (BELARUS - Tags: HEALTH SOCIETY) 

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. 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. REUTERS/Vasily Fedosenko (BELARUS - Tags: HEALTH SOCIETY) 

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. 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. REUTERS/Vasily Fedosenko (BELARUS - Tags: HEALTH SOCIETY) 

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. 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. REUTERS/Vasily Fedosenko (BELARUS - Tags: HEALTH SOCIETY)