LOS ANGELES (AP) - Scientists have discovered ancient supervolcanoes on Mars similar to the caldera that sits under Yellowstone National Park. Volcanoes previously have been spotted on Mars, which is known to have been volcanically active billions of years ago. What's different about this supervolcano network is that it was found in the Martian northern highlands, a place not known to be active in the past. Studying images from several spacecraft orbiting Mars, researchers at London's Natural History Museum and NASA found at least one massive volcano and evidence for several others that spewed huge clouds of lava and ash. The finding described in Thursday's issue of the journal Nature suggests that early Mars was more active than previously realized, and that such eruptions could have affected the red planet's climate and atmosphere.