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Smoke billows following the detonation of explosives placed by Syrian government forces inside a tunnel that was reportedly used by rebels in the northern Syrian city of Aleppo on May 19, 2015. AFP PHOTO / GEORGE OURFALIAN (Photo credit should read GEORGE OURFALIAN/AFP/Getty Images)

Smoke billows following the detonation of explosives placed by Syrian government forces inside a tunnel that was reportedly used by rebels in the northern Syrian city of Aleppo on May 19, 2015. AFP PHOTO / GEORGE OURFALIAN        (Photo credit should read GEORGE OURFALIAN/AFP/Getty Images)