Volcanologists have come a long way since Mt. Vesuvius buried Pompeii. Now they have at least a fighting chance of predicting a volcanic eruption. Yet researchers who specialize in volcanoes still don't fully understand the gaseous warning signs that spew out of active peaks. The composition of the smoke holds invaluable clues, but sampling the gases is dangerous and is often impossible as the eruption begins.

To provide a safer, continuous way of analyzing volcanic gases, physicists and geochemists at Los Alamos National Laboratory in New Mexico have used a remote sensing device to monitor plumes billowing from Popocatepetl, an active volcano 70 kilometers outside of Mexico City. The concentration of one of the gases depends on the temperature within the volcano, making the device "essentially a remote thermometer," says Steven Love, a physicist at Los Alamos.