But Pfanz was still mystified by one thing: if this area is so deadly, why did the priests in the Ploutonion not die too? He returned to Hierapolis the following year and this time he studied the concentrations of the gas at different times of the day. "We noticed that during the day, when it"s warm and sunny, the carbon dioxide quickly dissipates," he said. "But because carbon dioxide is heavier than air, at night when it"s cooler it pools in the arena, creating a lethal lake of gas at ground level."