Since 1994, the Popocatepetl volcano has intermittently shown signs of life. After a few months of relative calm, the volcano has again begun sputtering plumes of steam and gas. Full eruption such as this could be in Popocatepetl's future. Scientists at the National Center for the Prevention of Disasters, which monitors the 18,000-ft Mexican volcano, detected a 15-minute tremor Aug. 31.