Syria's civil war has been raging for four years, and there are no concrete signs it will end anytime soon.
Over 200,000 people have been killed and more than 11 million more have been displaced. The conflict is forcing combatants and civilians underground: Syrians are using massive cave complexes in the countryside as strongholds, meeting places, and defensive positions for battles.
Some of the caves are natural while others are artificial and recently excavated. They're used by combatants but also by civilians desperate to shield themselves and their families from the violence.
Looking inside these caves and tunnels shows us how brutal Syria's civil war has become.
Captions by Reuters and Christian Storm.
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, Syria, November 26, 2014. Residents are fleeing to ancient caves and cemeteries for safety during shelling from forces loyal to President Bashar Assad.
Many of the caves were once used during the Roman period but were abandoned and sealed for centuries. Some now serve as makeshift homes for families as they wait for the shelling to end.
Residents take shelter in a similar cave near Hama in this photo from 2012. The civil war has been raging in Syria for almost five years.
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