Archaeologists working on the site of an old Roman garrison town of Dura-Europos in Eastern Syria have found what is believed to be the earliest evidence of gas warfare. Around 254 AD, the Romans, in anticipation of an attack on the city, strengthened the defences. Amongst the many techniques used by the Persians in attacking the city, in about 256 AD, was the digging of many tunnels and mines to try and enter the city. Bitumen and sulphur crystals were ignited in some of these tunnels and the flow of air, aided possibly by bellows and chimneys, helped to generate and distribute their noxious fumes. In one such mine complex, the remains of 20 Roman soldiers have been found. It is believed that for so many men to have been killed in such a small space it must have required an agent that acted very quickly, such as the fumes from bitumen and sulphur. The soldiers would have been overcome in seconds and probably dead in minutes.