Efforts to mitigate the hazards presented by the accumulation of poisonous gas included the building of a large tephra wall to divert gases away from the town, and the digging of a trench to channel away the CO2 (carbon dioxide). These defences were only partially effective, as they relied on the assumption that the gases were produced at the vent, and flowed into the town from there. It is believed that as least some of the CO2 originated deep within the volcanic conduit and percolated through older volcanic rocks, rising directly into the town.