Every volcano works in a different way," explains Carlos Valdes Gonzalez, a scientist who monitors EI Popo. "What we're trying to learn here are the symptoms signaling that EI Popo will erupt."These include earthquakes, or any sign that the mountain's surface is changing or expanding.The hope is that scientists will be able to warn people in the surrounding areas so they have enough time to escape.A powerful eruption could displace over 20 million people-people whose lives can be saved if the warning is delivered early enough.