In the 1930s, archaeological excavations at Dura-Europos, Syria, uncovered an unusual battle scene. Skeletal remains buried near the ancient city's western walls indicated that a fight between Sasanian Persian soldiers and the occupying Roman side had taken place in a small mine shaft not large enough to stand in.

Nineteen Roman soldiers, as well as one Persian soldier, had been buried at the site in 256 CE. Piecing the clues together, archaeologists determined roughly what had happened there: The Sasanians wanted to make a gap in the Roman defenses. 

"Their objective was to create a large breach in the city defenses by undermining about 15 m (49 feet) of curtain wall and (to reduce the danger from defensive 'fire') the adjacent tower, making a gap wide enough for a column of troops to charge," archaeologist Simon James wrote of the attack in a 2011 paper.

The soldiers began digging a tunnel underneath the thick walls of the Roman military base, most likely beginning in a chamber tomb of a nearby necropolis, and aimed to bring down a tower. Unfortunately, on their approach, the Roman occupiers likely heard their underground activity and began to build a countermine.

"The Romans doubtless worked out that the Persian undermining operation within the mass of the rampart was likely accessed from its approach tunnel via a single vertical shaft," James continued. "If they could capture the Persian’s subrampart gallery and command the access shaft, the attack would be thwarted, and this appears to have been the counterminers’ practical objective".

According to previous assessments of the scene, the two mines met and there was a battle within the tunnels. Archaeologist Robert du Mesnil du Buisson, who had carried out the excavations, believed that the Sasanians had pushed the Romans back and slain them, before setting fire to the Roman section of the tunnel. This is how he explained deposits of sulfur and bitumen found in the tunnel.

However, when James reassessed the scene years later, something didn't quite add up. For one, the tunnels were too small to fight in, and the bodies didn't look right either.

"This wasn't a pile of people who had been crowded into a small space and collapsed where they stood," James told LiveScience. "This was a deliberate pile of bodies."

James came up with an alternative explanation: the Roman soldiers were killed in an early example of chemical warfare. According to James, the Sasarians likely knew of the Roman counterattack, just like they had known about their initial tunnel, and lay in wait. 

"As the Romans started to break through into the prop-filled void being created under the city wall, the Persians retreated into their approach tunnel behind their brazier and threw onto it some of the bitumen and sulphur crystals we know they had because they were using them, probably just minutes later, to set fire to the Roman tunnel," James explained in his paper. "These materials would have produced dense clouds of hot fumes, a deadly cocktail of oily hydrocarbon smoke incorporating carbon dioxide, lethal carbon monoxide, and—even nastier—sulphur dioxide gas."

James speculates that the soldiers pumped these gasses through the tunnel using bellows, as had been used before to smoke out enemies in similar battles. It's also possible that the relative heights of the tunnels – the Romans' being above – helped spread the gas in their direction, sealing their fate.

Within a few seconds, he writes, the Roman soldiers were choking to death and trying to stagger away from the "sulphurous clouds of Hell" as they followed them down the tunnel. Nineteen of the Romans died, and they were dragged, "some perhaps still alive", to create a blocking wall. One Sasarian soldier, it appears, was also killed by the fumes.

The study was published in the American Journal of Archaeology in 2011.