Syria's cave hideouts 

Rebel fighters sit outside caves which they dug to be used as shelters and to hide their tanks in al-Latamna town, in the northern countryside of Hama, Syria, March 4, 2015. REUTERS/Khalil Ashawi

A rebel fighter fires his weapon at the frontline against forces loyal to Syria's President Bashar al-Assad in al-Amriyah neighbourhood of Aleppo, October 11, 2014. REUTERS/Hosam Katan

A man is seen in a cave that is used as a shelter, under his house, in Kafruma, an area in the province of Idlib, October 26, 2012. REUTERS/Giath Taha

Free Syrian Army fighters are gathered inside a cave in Maaret al-Naaman village, in Idlib, October 17, 2013. REUTERS/Loubna Mrie

Mariam, a 9-year-old girl, carries her brother as she stands with other children at an underground ancient cemetery in Jabal al-Zawiya in the southern countryside of Idlib, November 26, 2014. Residents were using the ancient caves and cemeteries as underground shelters to hide in during shelling from forces loyal to President Bashar al-Assad. REUTERS/Khalil Ashawi

A rebel fighter of al-Badiya brigade in Jaysh al-Islam carries his weapon as he comes out from one of the caves used as shelter in al-Latamna town, in the northern countryside of Hama, March 4, 2015. REUTERS/Khalil Ashawi

A displaced girl poses inside an underground cave in Jabal al-Zawiya in the southern countryside of Idlib, November 26, 2014. REUTERS/Khalil Ashawi

A Free Syrian Army fighter prepares ammunition inside a cave in Maaret al-Naaman village, in Idlib, October 17, 2013. REUTERS/Loubna Mrie

Free Syrian Army fighters work on a computer inside a cave in Maaret al-Naaman village, in Idlib, October 17, 2013. REUTERS/Loubna Mrie

A Syrian family is pictured outside a cave they fled to in fear of shelling, at the Al-Arbaeen mountain in the Idlib countryside, September 22, 2013. REUTERS/Houssam Abo Dabak

Forces loyal to Syria's President Bashar al-Assad carry their weapons as they inspect a cave which they say belonged to rebel fighters in the village and hill of Al-Samsam in Hama countryside, after saying they have regained control of the area, September 13, 2014. REUTERS/George Ourfalian

A Free Syrian Army fighter with a weapon enters an underground cave in Maarat Al-Nouman, Idlib province, February 7, 2014. REUTERS/Fadi Mashan

A cave that used to belong to rebel fighters is pictured in Zur-Almsalq village, after forces loyal to Syria's President Bashar al-Assad said they have regained control of the village, along with Nasiriyah hill and Khattab city, in the Northern Hama countryside, September 15, 2014. REUTERS/George Ourfalian

Forces of Syria's President Bashar al-Assad inspect a base where caves were dug by rebel fighters in Zor al-Mahruqa village, after the forces said they had regained control of the area and its surrounding hills, in the Hama countryside, October 6, 2014. REUTERS/George Ourfalian

A Free Syrian Army fighter looks through a hole from a cave in Maaret al-Naaman village, in Idlib, October 17, 2013. REUTERS/Loubna Mrie

Free Syrian Army fighters sleep inside a cave in Maaret al-Naaman village, in Idlib, October 17, 2013. REUTERS/Loubna Mrie

Children dig in a cave under their house to use as a shelter in Kafruma, an area in the province of Idlib, October 26, 2012. REUTERS/Giath Taha

Forces loyal to Syria's President Bashar al-Assad carry their weapons as they inspect a cave which they say belonged to rebel fighters in the village and hill of Al-Samsam in Hama countryside, after saying they have regained control of the area, September 13, 2014. REUTERS/George Ourfalian

Syrian children sit outside a cave they fled to with their family members in fear of shelling, at the Al-Arbaeen mountain in the Idlib countryside, September 22, 2013. REUTERS/Houssam Abo Dabak

Residents dig tunnels and caves in mountains to be used as bomb shelters in Deir al-Zor, September 14, 2013. REUTERS/Mohamad Alderi

Residents take shelter in a cave near Hama, December 24, 2012. REUTERS/Samer Al-Hamwi/Shaam News Network

A displaced woman, standing with children, gestures as she talks inside an underground cave in Jabal al-Zawiya in the southern countryside of Idlib, November 26, 2014. REUTERS/Khalil Ashawi