Astronomers have spotted huge cloudlike plumes erupting from Mars - a phenomenon that scientists are at a loss to explain. The bright flares, which have now died away, towered higher than anything else observed in the Martian atmosphere. Their tops reached some 150 miles in altitude, more than twice as high as the highest Martian clouds, and they sprawled across 300 to 600 miles, researchers report in this week's Nature, a science journal. The researchers initially were skeptical, but we came to the conclusion that what we were seeing is actually real," says study co-author Antonio Garca Muoz, a planetary scientist at the European Space Agency. The plumes are "exceptional. It's difficult to come to terms with this." This scientific brainteaser first came to light in early 2012. "I don't think it's real. Basic physics says this can't occur," [Planetary scientist Todd Clancy of the Space Science Institute] says, adding that the conditions in Mars's upper atmosphere don't supply the necessary ingredients for clouds. In response, study co-author Agustin Snchez-Lavega, a planetary scientist and physics professor at the University of the Basque Country in Spain, notes that 19 different observers captured the strange eruptions. He considers the source of the formations "open to discussion," he says via e-mail.