The siege was notable for the early use of chemical weapons by the attacking Persian army. During the siege the attackers dug several underground shaft mines under the city walls. The Romans dug tunnels to reach the mines and fight the diggers underground. In one such tunnel, when the Romans broke through into the Sassanid tunnel the tunnelers ignited a mixture of sulfur and pitch, producing a cloud of sulfur dioxide which killed twenty Roman soldiers. Archaeologists excavated the scene in the 1930s. In 2009 tests showed the presence of sulfur dioxide inside the tunnel.