Sarin, a nerve agent, can kill in minutes, and chlorine and mustard gas have been known since World War I as a cruel weapon that "drowns people on land." Experts have warned that the reservoirs could fall into the hands of militant groups in Syria, as some facilities are not secured, or are not properly secured.
After the outbreak of the civil war in Syria, the Assad regime acknowledged the locations of 27 chemical weapons facilities and closed them down, but according to studies, it continued to import chemicals used in the manufacture of chemical weapons to the country.Some of the sites are probably hidden in caves or other places that are difficult to find using satellite imagery. (Source)